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Front matter

Version 2.3
The latest version is to be found here.
Please report errors, inaccuracies and suggestions to Richard Gruet (pqr at

23 Oct 2004, rev 8
updated by Richard Gruet
03 Oct 2003,
upgraded by Richard Gruet for Python 2.3
11 May 2003, rev 4
upgraded by Richard Gruet for Python 2.2 (restyled by Andrei)
7 Aug 2001
upgraded by Simon Brunning for Python 2.1
16 May 2001
upgraded by Richard Gruet and Simon Brunning for Python 2.0
18 Jun 2000
upgraded by Richard Gruet for Python 1.5.2
30 Oct 1995
created by Chris Hoffmann for Python 1.3
Color coding:
Features added in 2.3 since 2.2.
Features added in 2.2 since 2.1.
Features added in 2.1 since 2.0.
Originally based on:

Tip: From within the Python interpreter, use help or help(object).

Invocation Options

python[w] [-dEhiOQStuUvVWxX?] [-c command| scriptFile | - ] [args]
    (pythonw does not open a terminal/console; python does)
Invocation Options
Option Effect
-d Output parser debugging information (also PYTHONDEBUG=x)
-E Ignore environment variables (such as PYTHONPATH)
-h Print a help message and exit (formerly -?)
-i Inspect interactively after running script (also PYTHONINSPECT=x) and force prompts, even if stdin appears not to be a terminal.
-O Optimize generated bytecode (also PYTHONOPTIMIZE=x). Asserts are suppressed.
-OO Remove doc-strings in addition to the -O optimizations.
-Q arg Division options: -Qold (default), -Qwarn, -Qwarnall, -Qnew
-S Don't perform import site on initialization.
-t Issue warnings about inconsistent tab usage (-tt: issue errors).
-u Unbuffered binary stdout and stderr (also PYTHONUNBUFFERED=x).
-U Force Python to interpret all string literals as Unicode literals.
-v Verbose (trace import statements) (also PYTHONVERBOSE=x).
-V Print the Python version number and exit.
-W arg Warning control (arg is action:message:category:module:lineno)
-x Skip first line of source, allowing use of non-unix Forms of #!cmd
-X Disable class based built-in exceptions (for backward compatibility management of exceptions)
-c command Specify the command to execute (see next section). This terminates the option list (following options are passed as arguments to the command).
scriptFile The name of a python file (.py) to execute. Read from stdin.
- Program read from stdin (default; interactive mode if a tty).
args Passed to script or command (in sys.argv[1:])
  If no scriptFile or command, Python enters interactive mode.
  • Available IDEs in std distrib: IDLE (tkinter based, portable), Pythonwin (on Windows). Other free IDEs: BOA constructor, SPE.
  • Typical python module header :
  • #!/usr/bin/env python
    # -*- coding: latin1 -*-
    Since 2.3 the encoding of a Python source file must be declared as one of the two first lines (or defaults to 7 bits Ascii) [PEP-0263], with the format:
    # -*- coding: encoding -*-
    Std encodings are defined here, e.g. ISO-8859-1 (aka latin1), iso-8859-15 (latin9), UTF-8... Not all encodings supported, in particular UTF-16 is not supported.

Environment variables

Environment variables
Variable Effect
PYTHONHOME Alternate prefix directory (or prefix;exec_prefix). The default module search path uses prefix/lib
PYTHONPATH Augments the default search path for module files. The format is the same as the shell's $PATH: one or more directory pathnames separated by ':' or ';' without spaces around (semi-) colons!
On Windows first search for Registry key HKEY_LOCAL_MACHINE\Software\Python\PythonCore\x.y\PythonPath (default value). You may also define a key named after your application with a default string value giving the root directory path of your app.
Alternatively, you can create a text file in the Python home directory with a .pth extension, containing the path (one per line).
PYTHONSTARTUP If this is the name of a readable file, the Python commands in that file are executed before the first prompt is displayed in interactive mode (no default).
PYTHONDEBUG If non-empty, same as -d option
PYTHONINSPECT If non-empty, same as -i option
PYTHONOPTIMIZE If non-empty, same as -O option
PYTHONUNBUFFERED If non-empty, same as -u option
PYTHONVERBOSE If non-empty, same as -v option
PYTHONCASEOK If non-empty, ignore case in file/module names (imports)

Notable lexical entities


        and       del       for       is        raise
assert elif from lambda return
break else global not try
class except if or while
continue exec import pass yield
def finally in print
  • (List of keywords available in std module: keyword)
  • Illegitimate Tokens (only valid in strings): @ $ ?
  • A statement must all be on a single line. To break a statement over multiple lines use "\", as with the C preprocessor.
    Exception: can always break when inside any (), [], or {} pair, or in triple-quoted strings.
  • More than one statement can appear on a line if they are separated with semicolons (";").
  • Comments start with "#" and continue to end of line.


(letter | "_") (letter | digit | "_")*
  • Python identifiers keywords, attributes, etc. are case-sensitive.
  • Special forms: _ident (not imported by 'from module import *'); __ident__ (system defined name); __ident (class-private name mangling).

String literals

"a string enclosed by double quotes"
'another string delimited by single quotes and with a " inside'
'''a string containing embedded newlines and quote (') marks, can be delimited with triple quotes.'''
""" may also use 3- double quotes as delimiters """
u'a unicode string'
U"Another unicode string"
r'a raw string where \ are kept (literalized): handy for regular expressions and windows paths!'
R"another raw string"   -- raw strings cannot end with a \
ur'a unicode raw string'
UR"another raw unicode"
  • Use \ at end of line to continue a string on next line.
  • Adjacent strings are concatened, e.g. 'Monty' ' Python' is the same as 'Monty Python'.
  • u'hello' + ' world'  --> u'hello world'   (coerced to unicode)

String Literal Escapes
Escape Meaning
\newline Ignored (escape newline)
\\ Backslash (\)
\e Escape (ESC)
\v Vertical Tab (VT)
\' Single quote (')
\f Formfeed (FF)
\ooo char with octal value ooo
\" Double quote (")
\n Linefeed (LF)
\a Bell (BEL)
\r Carriage Return (CR)
\xhh char with hex value hh
\b Backspace (BS)
\t Horizontal Tab (TAB)
\uxxxx Character with 16-bit hex value xxxx (unicode only)
\Uxxxxxxxx Character with 32-bit hex value xxxxxxxx (unicode only)
\N{name} Character named in the Unicode database (unicode only), e.g. u'\N{Greek Small Letter Pi}' <=> u'\u03c0'.
(Conversely, in module unicodedata,'\u03c0') == 'GREEK SMALL LETTER PI')
\AnyOtherChar left as-is, including the backslash, e.g. str('\z') == '\\z'
  • NUL byte (\000) is not an end-of-string marker; NULs may be embedded in strings.
  • Strings (and tuples) are immutable: they cannot be modified.

Boolean constants

  • True
  • False

In 2.2, True and False are integers 1 and 0. Since 2.3, they are of new type bool.


  • Decimal integer: 1234, 1234567890546378940L  (or l)
  • Octal integer: 0177, 0177777777777777777L (begin with a 0)
  • Hex integer: 0xFF, 0XFFFFffffFFFFFFFFFFL (begin with 0x or 0X)
  • Long integer (unlimited precision): 1234567890123456L (ends with L or l) or long(1234)
  • Float (double precision): 3.14e-10, .001, 10., 1E3
  • Complex: 1J, 2+3J, 4+5j (ends with J or j, + separates (float) real and imaginary parts)

Integers and long integers are unified starting from release 2.2 (the L suffix is no longer required)


  • Strings (type str) of length 0, 1, 2 (see above)
    '', '1', "12", 'hello\n'
  • Tuples (type tuple) of length 0, 1, 2, etc:
    () (1,) (1,2) # parentheses are optional if len > 0
  • Lists (type list) of length 0, 1, 2, etc:
    [] [1] [1,2]
  • Indexing is 0-based. Negative indices (usually) mean count backwards from end of sequence.
  • Sequence slicing [starting-at-index : but-less-than-index [ : step]]. Start defaults to 0, end to len(sequence), step to 1.
      a = (0,1,2,3,4,5,6,7)
    a[3] == 3
    a[-1] == 7
    a[2:4] == (2, 3)
    a[1:] == (1, 2, 3, 4, 5, 6, 7)
    a[:3] == (0, 1, 2)
    a[:] == (0,1,2,3,4,5,6,7) # makes a copy of the sequence.
    a[::2] == (0, 2, 4, 6) # Only even numbers.
    a[::-1] = (7, 6, 5, 4, 3 , 2, 1, 0) # Reverse order.

Dictionaries (Mappings)

Dictionaries (type dict) of length 0, 1, 2, etc: {} {1 : 'first'} {1 : 'first',  'next': 'second'}

Operators and their evaluation order

Operators and their evaluation order
Highest Operator Comment
  , [...] {...} `...` Tuple, list & dict. creation; string conv.
s[i]  s[i:j] s.attr f(...) indexing & slicing; attributes, fct calls
+x, -x, ~x Unary operators
x**y Power
x*y  x/y  x%y mult, division, modulo
x+y  x-y addition, substraction
x<<y   x>>y Bit shifting
x&y Bitwise and
x^y Bitwise exclusive or
x|y Bitwise or
x<y  x<=y  x>y  x>=y  x==y x!=y  x<>y
x is y   x is not y
x in s   x not in s
not x boolean negation
x and y boolean and
x or y boolean or
Lowest lambda args: expr anonymous function
  • Alternate names are defined in module operator (e.g. __add__ and add for +)
  • Most operators are overridable

Basic types and their operations

Comparisons (defined between any types)

Comparison Meaning
< strictly less than
<= less than or equal to  
> strictly greater than  
>= greater than or equal to  
== equal to  
!= or <> not equal to  
is object identity
is not negated object identity
  • Comparison behavior can be overridden for a given class by defining special method __cmp__.
  • (1) X < Y < Z < W has expected meaning, unlike C
  • (2) Compare object identities (i.e. id(object)), not object values.


  • None is used as default return value on functions. Built-in single object with type NoneType. Might become a keyword in the future.
  • Input that evaluates to None does not print when running Python interactively.

Boolean operators

Boolean values and operators
Value or Operator Evaluates to
built-in bool(expr) True if expr is true, False otherwise.
None, numeric zeros, empty sequences and mappings considered False  
all other values considered True  
not x True if x is False, else False  
x or y if x is False then y, else x
x and y if x is False then x, else y
  • Truth testing behavior can be overridden for a given class by defining special method __nonzero__.
  • (1) Evaluate second arg only if necessary to determine outcome.

Numeric types

Floats, integers and long integers.

  • Floats (type float) are implemented with C doubles.
  • Integers (type int) are implemented with C longs (signed 32 bits, maximum value is sys.maxint)
  • Long integers (type long) have unlimited size (only limit is system resources).
  • Integers and long integers are unified starting from release 2.2 (the L suffix is no longer required). int() returns a long integer instead of raising OverflowError.

Operators on all numeric types

Operators on all numeric types
Operation Result
abs(x) the absolute value of x
int(x) x converted to integer
long(x) x converted to long integer
float(x) x converted to floating point
-x x negated
+x x unchanged
x + y the sum of x and y
x - y difference of x and y
x * y product of x and y
x / y true division of x by y: 1/2 -> 0.5 (1)
x // y floor division operator: 1//2 -> 0 (1)
x % y remainder of x / y
divmod(x, y) the tuple (x/y, x%y)
x ** y x to the power y (the same as pow(x,y))
  • (1) / is still a floor division (1/2 == 0) unless validated by a from __future__ import division.
  • classes may override methods __truediv__ and __floordiv__ to redefine these operators.

Bit operators on integers and long integers

Bit operators
Operation Result
~x the bits of x inverted
x ^ y bitwise exclusive or of x and y
x & y bitwise and of x and y
x | y bitwise or of x and y
x << n x shifted left by n bits
x >> n x shifted right by n bits

Complex Numbers

  • Type complex, represented as a pair of machine-level double precision floating point numbers.
  • The real and imaginary value of a complex number z can be retrieved through the attributes z.real and z.imag.

Numeric exceptions

raised on application of arithmetic operation to non-number
numeric bounds exceeded
raised when zero second argument of div or modulo op

Operations on all sequence types (lists, tuples, strings)

Operations on all sequence types
Operation Result
x in s True if an item of s is equal to x, else False
x not in s False if an item of s is equal to x, else True
s + t the concatenation of s and t  
s * n, n*s n copies of s concatenated
s[i] i'th item of s, origin 0
s[i: j]
s[i: j: step]
Slice of s from i (included) to j(excluded). Optional step value, possibly negative (default: 1).
(1), (2)
len(s) length of s  
min(s) smallest item of s
max(s) largest item of (s)
  • (1) if i or j is negative, the index is relative to the end of the string, ie len(s)+i or len(s)+j is substituted. But note that -0 is still 0.
  • (2) The slice of s from i to j is defined as the sequence of items with index k such that i <= k < j.
    If i or j is greater than len(s), use len(s). If i is omitted, use len(s). If i is greater than or equal to j, the slice is empty.
  • (3) For strings: before 2.3, x must be a single character string; Since 2.3, x in s is True if x is a substring of s.

Operations on mutable sequences (type list)

Operations on mutable sequences
Operation Result
s[i] =x item i of s is replaced by x  
s[i:j [:step]] = t slice of s from i to j is replaced by t  
del s[i:j[:step]] same as s[i:j] = []  
s.append(x) same as s[len(s) : len(s)] = [x]  
s.extend(x) same as s[len(s):len(s)]= x
s.count(x) return number of i's for which s[i] == x  
s.index(x[, start[, stop]]) return smallest i such that s[i]== x. start and stop limit search to only part of the list.
s.insert(i, x) same as s[i:i] = [x] if i >= 0. i == -1 inserts before the last element.  
s.remove(x) same as del s[s.index(x)]
s.pop([i]) same as x = s[i]; del s[i]; return x
s.reverse() reverse the items of s in place
s.sort([cmpFct ]) sort the items of s in place
(2), (3)
  • (1) Raises a ValueError exception when x is not found in s (i.e. out of range).
  • (2) The sort() method takes an optional argument specifying a comparison function of 2 arguments (list items) which should return -1, 0, or 1 depending on whether the 1st argument is considered smaller than, equal to, or larger than the 2nd argument. Note that this slows the sorting process down considerably.
  • (3) The sort() and reverse() methods modify the list in place for economy of space when sorting or reversing a large list. They don't return the sorted or reversed list to remind you of this side effect.
  • (4) The pop() method is not supported by mutable sequence types other than lists. The optional argument i defaults to -1, so that by default the last item is removed and returned.
  • (5) Raises a TypeError when x is not a list object.

Operations on mappings / dictionaries (type dict)

Operations on mappings
Operation Result
len(d) The number of items in d  
dict(**kw) Creates a dictionary. Use keyword list kw as initialization list.  
d.fromkeys(iterable, value=None) Class method to create a dictionary with keys provided by iterator, and all values set to value.  
d[k] The item of d with key k
d[k] = x Set d[k] to x  
del d[k] Remove d[k] from d
d.clear() Remove all items from d  
d.copy() A shallow copy of d  
k in d
True if d has key k, else False  
d.items() A copy of d's list of (key, item) pairs
d.keys() A copy of d's list of keys
d1.update(d2) for k, v in d2.items(): d1[k] = v
d.values() A copy of d's list of values
d.get(k, defaultval) The item of d with key k
d.setdefault(k[,defaultval]) d[k] if k in d, else defaultval (also setting it)
d.iteritems() Returns an iterator over (key, value) pairs.  
d.iterkeys() Returns an iterator over the mapping's keys.  
d.itervalues() Returns an iterator over the mapping's values.  
d.pop(k[, default]) Removes key k and returns the corresponding value. If key is not found, default is returned if given, otherwise KeyError is raised.  
d.popitem() Remove and return an arbitrary (key, value) pair from d
  • TypeError is raised if key is not acceptable
  • (1) KeyError is raised if key k is not in the map
  • (2) Keys and values are listed in random order
  • (3) d2 must be of the same type as d1
  • (4) Never raises an exception if k is not in the map, instead it returns defaultval. defaultval is optional, when not provided and k is not in the map, None is returned.
  • (5) Never raises an exception if k is not in the map, instead it returns defaultVal, and adds k to map with value defaultVal. defaultVal is optional. When not provided and k is not in the map, None is returned and added to map.

Operations on strings (type str)

These string methods largely (but not completely) supersede the functions available in the string module.
The str and unicode types share a common base class basestring.

Operations on strings
Operation Result
s.capitalize() Returns a copy of s with only its first character capitalized. Returns a copy of s centered in a string of length width.
s.count(sub[ ,start[,end]]) Returns the number of occurrences of substring sub in string s.
s.encode([ encoding[,errors]]) Returns an encoded version of s. Default encoding is the current default string encoding.
s.endswith(suffix [,start[,end]]) Returns True if s ends with the specified suffix, otherwise return false.
s.expandtabs([ tabsize]) Returns a copy of s where all tab characters are expanded using spaces.
s.find(sub[ ,start[,end]]) Returns the lowest index in s where substring sub is found. Return -1 if sub is not found.
s.index(sub[ ,start[,end]]) like find(), but raises ValueError when the substring is not found.
s.isalnum() Returns True if all characters in s are alphanumeric, False otherwise.
s.isalpha() Returns True if all characters in s are alphabetic, False otherwise.
s.isdigit() Returns True if all characters in s are digit characters, False otherwise.
s.islower() Returns True if all characters in s are lowercase, False otherwise.
s.isspace() Returns True if all characters in s are whitespace characters, False otherwise.
s.istitle() Returns True if string s is a titlecased string, False otherwise.
s.isupper() Returns True if all characters in s are uppercase, False otherwise.
separator.join(seq) Returns a concatenation of the strings in the sequence seq, separated by string separator, e.g.: ", ".join(['A', 'B', 'C']) -> "A, B, C"
s.ljust(width) Returns s left justified in a string of length width.
(1), (8)
s.lower() Returns a copy of s converted to lowercase.
s.lstrip([chars] ) Returns a copy of s with leading chars(default: whitespaces) removed.
s.replace(old, new[, maxCount =-1]) Returns a copy of s with the first maxCount (-1: unlimited) occurrences of substring old replaced by new.
s.rfind(sub[ , start[, end]]) Returns the highest index in s where substring sub is found. Returns -1 if sub is not found.
s.rindex(sub[ , start[, end]]) like rfind(), but raises ValueError when the substring is not found.
s.rjust(width) Returns s right justified in a string of length width.
(1), (8)
s.rstrip([chars] ) Returns a copy of s with trailing chars(default: whitespaces) removed.
s.split([ separator[, maxsplit]]) Returns a list of the words in s, using separator as the delimiter string.
s.splitlines([ keepends]) Returns a list of the lines in s, breaking at line boundaries.
s.startswith(prefix [, start[, end]]) Returns True if s starts with the specified prefix, otherwise returns False. Negative numbers may be used for start and end
s.strip([chars] ) Returns a copy of s with leading and trailing chars(default: whitespaces) removed.
s.swapcase() Returns a copy of s with uppercase characters converted to lowercase and vice versa.
s.title() Returns a titlecased copy of s, i.e. words start with uppercase characters, all remaining cased characters are lowercase.
s.translate(table [, deletechars]) Returns a copy of s mapped through translation table table.
s.upper() Returns a copy of s converted to uppercase.
s.zfill(width) Returns the numeric string left filled with zeros in a string of length width.  
  • (1) Padding is done using spaces.
  • (2) If optional argument start is supplied, substring s[start:] is processed. If optional arguments start and end are supplied, substring s[start:end] is processed.
  • (3) Optional argument errors may be given to set a different error handling scheme. The default for errors is 'strict', meaning that encoding errors raise a ValueError. Other possible values are 'ignore' and 'replace'.
  • (4) If optional argument tabsize is not given, a tab size of 8 characters is assumed.
  • (5) Returns False if string s does not contain at least one character.
  • (6) Returns False if string s does not contain at least one cased character.
  • (7) A titlecased string is a string in which uppercase characters may only follow uncased characters and lowercase characters only cased ones.
  • (8) s is returned if width is less than len(s).
  • (9) If the optional argument maxsplit is given, only the first maxsplit occurrences are replaced.
  • (10) If sep is not specified or None, any whitespace string is a separator. If maxsplit is given, at most maxsplit splits are done.
  • (11) Line breaks are not included in the resulting list unless keepends is given and true.
  • (12) table must be a string of length 256. All characters occurring in the optional argument deletechars are removed prior to translation.

String formatting with the % operator

formatString % args --> evaluates to a string
  • formatString uses C printf format codes : %, c, s, i, d, u, o, x, X, e, E, f, g, G, r (details below).
  • Width and precision may be a * to specify that an integer argument gives the actual width or precision.
  • The flag characters -, +, blank, # and 0 are understood (details below).
  • %s will convert any type argument to string (uses str() function)
  • args may be a single arg or a tuple of args
    '%s has %03d quote types.' % ('Python', 2)  == 'Python has 002 quote types.'
  • Right-hand-side can also be a mapping:
    a = '%(lang)s has %(c)03d quote types.' % {'c':2, 'lang':'Python'}
    (vars() function very handy to use on right-hand-side)

Format codes
Code Meaning
d Signed integer decimal.
i Signed integer decimal.
o Unsigned octal.
u Unsigned decimal.
x Unsigned hexadecimal (lowercase).
X Unsigned hexadecimal (uppercase).
e Floating point exponential format (lowercase).
E Floating point exponential format (uppercase).
f Floating point decimal format.
F Floating point decimal format.
g Same as "e" if exponent is greater than -4 or less than precision, "f" otherwise.
G Same as "E" if exponent is greater than -4 or less than precision, "F" otherwise.
c Single character (accepts integer or single character string).
r String (converts any python object using repr()).
s String (converts any python object using str()).
% No argument is converted, results in a "%" character in the result. (The complete specification is %%.)
Conversion flag characters
Flag Meaning
# The value conversion will use the ``alternate form''.
0 The conversion will be zero padded.
- The converted value is left adjusted (overrides "-").
  (a space) A blank should be left before a positive number (or empty string) produced by a signed conversion.
+ A sign character ("+" or "-") will precede the conversion (overrides a "space" flag).

File objects

(Type file). Created with built-in functions open() [preferred] or its alias file(). May be created by other modules' functions as well.
Unicode file names are now supported for all functions accepting or returning file names (open, os.listdir, etc...).

Operators on file objects

File operations
Operation Result
f.close() Close file f.
f.fileno() Get fileno (fd) for file f.
f.flush() Flush file f's internal buffer.
f.isatty() 1 if file f is connected to a tty-like dev, else 0.[size]) Read at most size bytes from file f and return as a string object. If size omitted, read to EOF.
f.readline() Read one entire line from file f. The returned line has a trailing \n, except possibly at EOF.
f.readlines() Read until EOF with readline() and return a list of lines read.
f.xreadlines() Return a sequence-like object for reading a file line-by-line without reading the entire file into memory. From 2.2, use rather: for line in f (see below).
for line in f: do something... Iterate over the lines of a file (using readline)[, whence=0]) Set file f's position, like "stdio's fseek()".
whence == 0 then use absolute indexing.
whence == 1 then offset relative to current pos.
whence == 2 then offset relative to file end.
f.tell() Return file f's current position (byte offset).
f.write(str) Write string to file f.
f.writelines(list) Write list of strings to file f. No EOL are added.

File Exceptions

End-of-file hit when reading (may be raised many times, e.g. if f is a tty).
Other I/O-related I/O operation failure


The Module sets define a new set data type implementation, via class Set for mutable sets, and class ImmutableSet for non mutable sets. Sets are unordered collections of unique (non duplicate) elements. Elements must be hashable.

Main Set operations
Operation Result
Set/ImmutableSet([iterable=None]) Builds a Set or ImmutableSet from the given iterable (default: empty), e.g. Set([1,2,3]).
len(s) Cardinality of set s.
elt in s True if element elt belongs to set s.
s1.issubset(s2) True if every element in s1 is in s2.
s1.issuperset(s2) True if every element in s2 is in s1.
s.add(elt) Adds element elt to set s (if it doesn't already exist).
s.remove(elt) Removes element elt from set s. KeyError if element not found.
s1.intersection(s2) Returns a new Set with elements common to s1 and s2. Also noted s1&s2.
s1.union(s2) Returns a new Set with elements from both s1 and s2. Also noted s1|s2.
s1.difference(s2) Returns a new Set with elements in s1 but not in s2. Also noted s1-s2.
s1.symmetric_difference(s2) Returns a new Set with elements in either s1 or s2 but not both. Also noted s1^s2.
s.copy() Returns a shallow copy of set s.

Advanced Types

- See manuals for more details -
  • Module objects
  • Class objects
  • Class instance objects
  • Type objects (see module: types)
  • File objects (see above)
  • Slice objects
  • Ellipsis object, used by extended slice notation (unique, named Ellipsis)
  • Null object (unique, named None)
  • XRange objects
  • Callable types:
    • User-defined (written in Python):
      • User-defined Function objects
      • User-defined Method objects
    • Built-in (written in C):
      • Built-in Function objects
      • Built-in Method object
  • Internal Types:
    • Code objects (byte-compile executable Python code: bytecode)
    • Frame objects (execution frames)
    • Traceback objects (stack trace of an exception)


Statement Result
pass Null statement
del name[, name]* Unbind name(s) from object. Object will be indirectly (and automatically) deleted only if no longer referenced.
print[>> fileobject,] [s1 [, s2 ]* [,] Writes to sys.stdout, or to fileobject if supplied. Puts spaces between arguments. Puts newline at end unless statement ends with comma. Print is not required when running interactively, simply typing an expression will print its value, unless the value is None.
exec x [in globals [, locals]] Executes x in namespaces provided. Defaults to current namespaces. x can be a string, file object or a function object.
callable(value,... [id=value] , [*args], [**kw]) Call function callable with parameters. Parameters can be passed by name or be omitted if function defines default values. E.g. if callable is defined as "def callable(p1=1, p2=2)"
"callable()" <=> "callable(1, 2)"
"callable(10)" <=> "callable(10, 2)"
"callable(p2=99)" <=> "callable(1, 99)"
*args is a tuple of positionalarguments.
**kw is a dictionary of keyword arguments.
yield expression (Only used within the body of a generator function, outside a try of a try..finally). "Returns" the evaluated expression.

Assignment operators

Assignment operators
Operator Result
a = b Basic assignment - assign object b to label a
a += b Roughly equivalent to a = a + b
a -= b Roughly equivalent to a = a - b
a *= b Roughly equivalent to a = a * b
a /= b Roughly equivalent to a = a / b
a //= b Roughly equivalent to a = a // b
a %= b Roughly equivalent to a = a % b
a **= b Roughly equivalent to a = a ** b
a &= b Roughly equivalent to a = a & b
a |= b Roughly equivalent to a = a | b
a ^= b Roughly equivalent to a = a ^ b
a >>= b Roughly equivalent to a = a >> b
a <<= b Roughly equivalent to a = a << b
  • (1) Can unpack tuples, lists, and strings:
    first, second = a[0:2]
    [f, s] = range(2)
    Tip: x,y = y,x swaps x and y.
  • (2) Not exactly equivalent - a is evaluated only once. Also, where possible, operation performed in-place - a is modified rather than replaced.

Control Flow statements

Control flow statements
Statement Result
if condition:
[elif condition:   suite]*
Usual if/else if/else statement
while condition:
Usual while statement. The else suite is executed after loop exits, unless the loop is exited with break.
for element in sequence:
Iterates over sequence, assigning each element to element. Use built-in range function to iterate a number of times. The else suite is executed at end unless loop exited with break.
break Immediately exits for or while loop.
continue Immediately does next iteration of for or while loop.
return [result] Exits from function (or method) and returns result (use a tuple to return more than one value). If no result given, then returns None.

Exception statements

Exception statements
Statement Result
assert expr[, message] expr is evaluated. if false, raises exception AssertionError with message. Before 2.3, inhibited if __debug__ is 0.
[except [exception [, value]:
Statements in suite1 are executed. If an exception occurs, look in except clauses for matching exception. If matches or bare except, execute suite of that clause. If no exception happens, suite in else clause is executed after suite1. If exception has a value, it is put in variable value. exception can also be a tuple of exceptions, e.g. except(KeyError, NameError), val: print val.
Statements in suite1 are executed. If no exception, execute suite2 (even if suite1 is exited with a return,break or continue statement). If exception did occur, executes suite2 and then immediately re-raises exception.
raise exceptionInstance Raises an instance of a class derived from Exception (preferred form of raise).
raise exceptionClass [,value [, traceback]] Raises exception of given class exceptionClass with optional value value. Arg traceback specifies a traceback object to use when printing the exception's backtrace.
raise A raise statement without arguments re-raises the last exception raised in the current function.
  • An exception is an instance of an exception class (before 2.0, it may also be a mere string).
  • Exception classes must be derived from the predefined class: Exception, e.g.:
    class TextException(Exception): pass
    if bad:
    raise TextException()
    except Exception:
    print 'Oops' # This will be printed because TextException is a subclass of Exception
  • When an error message is printed for an unhandled exception, the class name is printed, then a colon and a space, and finally the instance converted to a string using the built-in function str().
  • All built-in exception classes derives from StandardError, itself derived from Exception.

Name Space Statements

Imported module files must be located in a directory listed in the Python path (sys.path). Since 2.3, they may reside in a zip file [e.g. sys.path.insert(0, "")].

Packages (>1.5): a package is a name space which maps to a directory including module(s) and the special initialization module (possibly empty).
Packages/directories can be nested. You address a module's symbol via [package.[package...].module.symbol.
[1.51: On Mac & Windows, the case of module file names must now match the case as used in the import statement]

Name space statements
Statement Result
import module1 [as name1] [, module2]* Imports modules. Members of module must be referred to by qualifying with [package.]module name, e.g.:
import sys; print sys.argv
import package1.subpackage.module
module1 renamed as name1, if supplied.
from module import name1 [as othername1] [, name2]* Imports names from module module in current namespace.
from sys import argv; print argv
from package1 import module;
from package1.module import foo; foo()
name1 renamed as othername1, if supplied.
from module import * Imports all names in module, except those starting with "_" Use sparsely, beware of name clashes!
from sys import *; print argv
from package.module import *; print x
Only legal at the top level of a module.
If module defines an __all__ attribute, only names listed in __all__ will be imported.

NB: "from package import *" only imports the symbols defined in the package's file, not those in the package's modules !
global name1 [, name2] Names are from global scope (usually meaning from module) rather than local (usually meaning only in function).
E.g. in function without global statements, assuming "x" is name that hasn't been used in function or module so far:
- Try to read from "x" -> NameError
- Try to write to "x" -> creates "x" local to function
If "x" not defined in fct, but is in module, then: - Try to read from "x", gets value from module
- Try to write to "x", creates "x" local to fct
But note "x[0]=3" starts with search for "x", will use to global "x" if no local "x".

Function Definition

def func_id ([param_list]):
Creates a function object and binds it to name func_id.
param_list ::= [id [, id]*]
id ::= value | id = value | *id | **id
Args are passed by value.Thus only args representing a mutable object can be modified (are inout parameters). Use a tuple to return more than one value.

 def test (p1, p2 =5+3, *args, **kwargs):
  • args with "=" have a default value (evaluated at function definition time).
  • If arg list has "*args" then args is assigned a tuple of all remaining non-keywords args passed to the function.
  • If list has "**kwargs" then kwargs is assigned a dictionary of all extra arguments passed as keywords.
  • args and kwargs are common names but other names may be used as well.

Class Definition

class className [(super_class1 [, super_class2]*)]:

Creates a class object and assigns it name className.
suite may contain local "defs" of class methods and assignments to class attributes.

class MyClass (class1, class2): ...
Creates a class object inheriting from both class1 and class2. Assigns new class object to name "MyClass".
class MyClass: ...
Creates a base class object (inheriting from nothing). Assigns new class object to name "MyClass".
class MyClass (object): ...
Creates a new-style class/type (inheriting from object makes a class a new-style class). Assigns new class object to name "MyClass".

  • First arg to class instance methods (operations) is always the target instance object, called 'self' by convention.
  • Special method __init__() is called when instance is created.
  • Special method __del__() called when no more reference to object.
  • Create instance by "calling" class object, possibly with arg (thus instance=apply(aClassObject, args...) creates an instance!)
  • Before 2.2 it was not possible to subclass built-in classes like list, dict (you had to "wrap" them, using UserDict & UserList modules); since 2.2 you can subclass them directly (see Types/Classes unification).

class c (c_parent):
def __init__(self, name): = name
def print_name(self):
print "I'm",
def call_parent(self):

instance = c('tom')
"I'm tom"

Call parent's super class by accessing parent's method directly and passing "self" explicitly (see "call_parent" in example above).
Many other special methods available for implementing arithmetic operators, sequence, mapping indexing, etc...

Types / classes unification

Base types int, float, str, list, tuple, dict and file now (2.2) behave like classes derived from base class object, and may be subclassed:
x = int(2) # built-in cast function now a constructor for base type
y = 3 # <=> int(3) (litterals are instances of new base types)
print type(x), type(y) # int, int

assert isinstance(x, int) # replaces isinstance(x, types.IntType)

assert issubclass(int, object) # base types derive from base class 'object'.
s = "hello" # <=> str("hello")
assert isinstance(s, str)

f = 2.3 # <=> float(2.3)
class MyInt(int): pass # may subclass base types
x,y = MyInt(1), MyInt("2")

print x, y, x+y # => 1,2,3

class MyList(list): pass

l = MyList("hello")

print l # ['h', 'e', 'l', 'l', 'o']
New-style classes extends object.Old-style classes don't.

Documentation Strings

Modules, classes and functions may be documented by placing a string literal by itself as the first statement in the suite. The documentation can be retrieved by getting the '__doc__' attribute from the module, class or function.

class C:
"A description of C"
def __init__(self):
"A description of the constructor"
# etc.

c.__doc__ == "A description of C".
c.__init__.__doc__ == "A description of the constructor"


  • An iterator enumerates elements of a collection. It is an object with a single method next() returning the next element or raising StopIteration.
  • You get an iterator on obj via the new built-in function iter(obj), which calls obj.__class__.__iter__().
  • A collection may be its own iterator by implementing both __iter__() and next().
  • Built-in collections (lists, tuples, strings, dict) implement __iter__(); dictionaries (maps) enumerate their keys; files enumerates their lines.
  • Python uses implicitely iterators wherever it has to loop :
    • for elt in collection:
    • if elt in collection:
    • when assigning tuples: x,y,z= collection


  • A generator is a function that retains its state between 2 calls and produces a new value at each invocation. The values are returned (one at a time) using the new keyword yield, while return or raise StopIteration() are used to notify the end of values.
  • A typical use is the production of IDs, names, or serial numbers.
  • To use a generator: call the generator function to get a generator object, then call to get the next value until StopIteration is raised.
  • In 2.2, feature needs to be enabled by the statement: from __future__ import generators (not required since 2.3+)

def genID(initialValue=0):
v = initialValue
while v < initialValue + 1000:
yield "ID_%05d" % v
v += 1
return # or: raise StopIteration()

generator = genID() # Create a generator
for i in range(10): # Generates 10 values

Descriptors / Attribute access

  • Descriptors are objects implementing at least the first of these 3 methods representing the descriptor protocol:
    • __get__(self, obj, type=None) --> value
    • __set__(self, obj, value)
    • __delete__(self, obj)
    Python now transparently uses descriptors to describe and access the attributes and methods of new-style classes (i.e. derived from object). [more info])
  • Built-in descriptors now allow to define:
    • Static methods : Use staticmethod(f) to make method f(x) static (unbound).
    • Class methods: like a static but takes the Class as 1st argument => Use f = classmethod(f) to make method f(theClass, x) a class method.
    • Properties : A property is an instance of the new built-in type property, which implements the descriptor protocol for attributes => Use propertyName = property(getter=None, setter=None, deleter=None, description=None) to define a property inside or outside a class. Then access it as propertyName or obj.propertyName
    • Slots. New style classes can define a class attribute __slots__ to constrain the list of assignable attribute names, to avoid typos (which is normally not detected by Python and leads to the creation of new attributes), e.g. __slots__ = ('x', 'y')
      Note: According to recent discussions, the real purpose of slots seems still unclear (optimization?), and their use should probably be discouraged.


lambda [param_list]: returnedExpr
Creates an anonymous function.
returnedExpr must be an expression, not a statement (e.g., not "if xx:...", "print xxx", etc.) and thus can't contain newlines. Used mostly for filter(), map(), reduce() functions, and GUI callbacks.

List comprehensions
result = [expression for item1 in sequence1  [if condition1]
[for item2 in sequence2 ... for itemN in sequenceN]
is equivalent to:
result = []
for item1 in sequence1:
for item2 in sequence2:
for itemN in sequenceN:
if (condition1) and further conditions:

Nested scopes
In 2.2 nested scopes no longer need to be specially enabled by a from __future__ import nested_scopes directive, and are now always present.

Built-In Functions

Built-In Functions
Function Result
__import__(name[, globals[,locals[,from list]]]) Imports module within the given context (see library reference for more details)
abs(x) Returns the absolute value of the number x.
apply(f, args[, keywords]) Calls func/method f with arguments args and optional keywords.
buffer(object[, offset[, size]]) Returns a Buffer from a slice of object, which must support the buffer call interface (string, array, buffer).
callable(x) Returns True if x callable, else False.
chr(i) Returns one-character string whose ASCII code isinteger i.
classmethod(function) Returns a class method for function. A class method receives the class as implicit first argument, just like an instance method receives the instance. To declare a class method, use this idiom:

class C:
  def f(cls, arg1, arg2, ...): ...
  f = classmethod(f)

Then call it on the class C.f() or on an instance C().f(). The instance is ignored except for its class. If a class method is called for a derived class, the derived class object is passed as the implied first argument.
cmp(x,y) Returns negative, 0, positive if x <, ==, > to y respectively.
coerce(x,y) Returns a tuple of the two numeric arguments converted to a common type.
compile(string, filename, kind[, flags[, dont_inherit]]) Compiles string into a code object. filename is used in error message, can be any string. It is usually the file from which the code was read, or eg. '<string>' if not read from file. kind can be 'eval' if string is a single stmt, or 'single' which prints the output of expression statements that evaluate to something else than None, or be 'exec'. New args flags and dont_inherit concern future statements.
complex(real[, image]) Creates a complex object (can also be done using J or j suffix, e.g. 1+3J).
delattr(obj, name) Deletes the attribute named name of object obj <=> del
dict([mapping-or-sequence]) Returns a new dictionary initialized from the optional argument (or an empty dictionary if no argument). Argument may be a sequence (or anything iterable) of pairs (key,value).
dir([object]) Without args, returns the list of names in the current local symbol table. With a module, class or class instance object as arg, returns the list of names in its attr. dictionary.
divmod(a,b) Returns tuple (a/b, a%b)
enumerate(iterable) Iterator returning pairs (index, value) of iterable, e.g. List(enumerate('Py')) -> [(0, 'P'), (1, 'y')].
eval(s[, globals[, locals]]) Evaluate string s in (optional) globals, locals contexts. s must have no NUL's or newlines. s can also be a code object.
x = 1; incr_x = eval('x + 1')
execfile(file[, globals[,locals]]) Executes a file without creating a new module, unlike import.
file(filename[,mode[,bufsize]]) Opens a file and returns a new file object. Alias for open.
filter(function,sequence) Constructs a list from those elements of sequence for which function returns true. function takes one parameter.
float(x) Converts a number or a string to floating point.
getattr(object,name[,default])) Gets attribute called name from object, e.g. getattr(x, 'f') <=> x.f). If not found, raises AttributeError or returns default if specified.
globals() Returns a dictionary containing the current global variables.
hasattr(object, name) Returns true if object has an attribute called name.
hash(object) Returns the hash value of the object (if it has one).
help([object]) Invokes the built-in help system. No argument -> interactive help; if object is a string (name of a module, function, class, method, keyword, or documentation topic), a help page is printed on the console; otherwise a help page on object is generated.
hex(x) Converts a number x to a hexadecimal string.
id(object) Returns a unique integer identifier for object.
input([prompt]) Prints prompt if given. Reads input and evaluates it. Uses line editing / history if module readline available.
int(x[, base]) Converts a number or a string to a plain integer. Optional base parameter specifies base from which to convert string values.
intern(aString) Enters aString in the table of interned strings and returns the string. Before 2.3, interned strings were 'immortals' (never garbage collected). This is no longer true in 2.3+.
isinstance(obj, classInfo) Returns true if obj is an instance of class classInfo or an object of type classInfo (classInfo may also be a tuple of classes or types). If issubclass(A,B) then isinstance(x,A) => isinstance(x,B)
issubclass(class1, class2) Returns true if class1 is derived from class2 (or if class1 is class2).
iter(obj[,sentinel]) Returns an iterator on obj. If sentinel is absent, obj must be a collection implementing either __iter__() or __getitem__(). If sentinel is given, obj will be called with no arg; if the value returned is equal to sentinel, StopIteration will be raised, otherwise the value will be returned. See Iterators.
len(obj) Returns the length (the number of items) of an object (sequence, dictionary, or instance of class implementing __len__).
list(sequence) Converts sequence into a list. If already a list, returns a copy of it.
locals() Returns a dictionary containing current local variables.
long(x[, base]) Converts a number or a string to a long integer. Optional base parameter specifies the base from which to convert string values.
map(function, list, ...) Applies function to every item of list and returns a list of the results. If additional arguments are passed, function must take that many arguments and they are given to function on each call.
max(seq[, args...]) With a single argument seq, returns the largest item of a non-empty sequence (such as a string, tuple or list). With more than one argument, returns the largest of the arguments.
min(seq[, args...]) With a single argument seq, returns the smallest item of a non-empty sequence (such as a string, tuple or list). With more than one argument, returns the smallest of the arguments.
oct(x) Converts a number to an octal string.
open(filename [, mode='r', [bufsize]]) Returns a new file object. See also alias file().
  • filename is the file name to be opened
  • mode indicates how the file is to be opened:
    • 'r' for reading
    • 'w' for writing (truncating an existing file)
    • 'a' opens it for appending
    • '+' (appended to any of the previous modes) open the file for updating (note that 'w+'truncates the file)
    • 'b' (appended to any of the previous modes) open the file in binary mode
    • 'U' (or 'rU') open the file for reading in Universal Newline mode: all variants of EOL (CR, LF, CR+LF) will be translated to a single LF ('\n').
  • bufsize is 0 for unbuffered, 1 for line-buffered, negative for sys-default, all else, of (about) given size.
ord(c) Returns integer ASCII value of c (a string of len 1). Works with Unicode char.
pow(x, y [, z]) Returns x to power y [modulo z]. See also ** operator.
range(start [,end [, step]]) Returns list of ints from >= start and < end.
With 1 arg, list from 0..arg-1
With 2 args, list from start..end-1
With 3 args, list from start up to end by step
raw_input([prompt]) Prints prompt if given, then reads string from std input (no trailing \n). See also input().
reduce(f, list [, init]) Applies the binary function f to the items of list so as to reduce the list to a single value. If init is given, it is "prepended" to list.
reload(module) Re-parses and re-initializes an already imported module. Useful in interactive mode, if you want to reload a module after fixing it. If module was syntactically correct but had an error in initialization, must import it one more time before calling reload().
repr(object) Returns a string containing a printable and if possible evaluable representation of an object. <=> `object` (using backquotes). Class redefinable (__repr__). See also str()
round(x, n=0) Returns the floating point value x rounded to n digits after the decimal point.
setattr(object, name, value) This is the counterpart of getattr().setattr(o, 'foobar', 3) <=> o.foobar = 3. Creates attribute if it doesn't exist!
slice([start,] stop[, step]) Returns a slice object representing a range, with R/O attributes: start, stop, step.
staticmethod(function) Returns a static method for function. A static method does not receive an implicit first argument. To declare a static method, use this idiom:

class C:
  def f(arg1, arg2, ...): ...
  f = staticmethod(f)

Then call it on the class C.f() or on an instance C().f(). The instance is ignored except for its class.
str(object) Returns a string containing a nicely printable representation of an object. Class overridable (__str__). See also repr().
sum(iterable[, start=0]) Returns the sum of a sequence of numbers (not strings), plus the value of parameter. Returns start when the sequence is empty.
super( type[, object-or-type]) Returns the superclass of type. If the second argument is omitted the super object returned is unbound. If the second argument is an object, isinstance(obj, type) must be true. If the second argument is a type, issubclass(type2, type) must be true. Typical use :
class C(B):
  def meth(self, arg):
    super(C, self).meth(arg)
tuple([sequence]) Creates an empty tuple or a tuple with same elements as sequence. sequence may be a sequence, a container that supports iteration, or an iterator object. If sequence is already a tuple, return itself (not a copy).
type(obj) Returns a type object [see module types] representing the type of obj. Example: import types if type(x) == types.StringType: print 'It is a string'. NB: it is better to use instead: if isinstance(x, types.StringType)...
unichr(code) Returns a unicode string 1 char long with given code.
unicode(string[, encoding[,error]]]) Creates a Unicode string from a 8-bit string, using the given encoding name and error treatment ('strict', 'ignore',or 'replace'}. For objects which provide a __unicode__() method, it will call this method without arguments to create a Unicode string.
vars([object]) Without arguments, returns a dictionary corresponding to the current local symbol table. With a module, class or class instance object as argument, returns a dictionary corresponding to the object's symbol table. Useful with the "%" string formatting operator.
xrange(start [, end [, step]]) Like range(), but doesn't actually store entire list all at once. Good to use in "for" loops when there is a big range and little memory.
zip(seq1[, seq2,...]) Returns a list of tuples where each tuple contains the nth element of each of the argument sequences.

Built-In Exception classes

The mother of all exceptions. exception.args is a tuple of the arguments passed to the constructor.
  • StopIteration
    Raised by an iterator's next() method to signal that there are no further values.
  • SystemExit
    On sys.exit()
  • Warning
    Base class for warnings (see module warning)
    • UserWarning
      Warning generated by user code.
    • PendingDeprecationWarning
      Warning about future deprecated code.
    • DeprecationWarning
      Warning about deprecated code.
    • SyntaxWarning
      Warning about dubious syntax.
    • RuntimeWarning
      Warning about dubious runtime behavior.
  • StandardError
    Base class for all built-in exceptions; derived from Exception root class.
    • ArithmeticError
      Base class for arithmetic errors.
      • FloatingPointError
        When a floating point operation fails.
      • OverflowError
        On excessively large arithmetic operation.
      • ZeroDivisionError
        On division or modulo operation with 0 as 2nd argument.
    • AssertionError
      When an assert statement fails.
    • AttributeError
      On attribute reference or assignment failure
  • EnvironmentError [new in 1.5.2]
    On error outside Python; error arg. tuple is (errno, errMsg...)
    • IOError [changed in 1.5.2]
      I/O-related operation failure.
    • OSError [new in 1.5.2]
      Used by the os module's os.error exception.
      • WindowsError
        When a Windows-specific error occurs or when the error number does not correspond to an errno value.
  • EOFError
    Immediate end-of-file hit by input() or raw_input()
  • ImportError
    On failure of import to find module or name.
  • KeyboardInterrupt
    On user entry of the interrupt key (often `CTRL-C')
  • LookupError
    base class for IndexError, KeyError
    • IndexError
      On out-of-range sequence subscript
    • KeyError
      On reference to a non-existent mapping (dict) key
  • MemoryError
    On recoverable memory exhaustion
  • NameError
    On failure to find a local or global (unqualified) name.
    • UnboundLocalError
      On reference to an unassigned local variable.
  • ReferenceError
    On attempt to access to a garbage-collected object via a weak reference proxy.
  • RuntimeError
    Obsolete catch-all; define a suitable error instead.
    • NotImplementedError [new in 1.5.2]
      On method not implemented.
  • SyntaxError
    On parser encountering a syntax error
    • IndentationError
      On parser encountering an indentation syntax error
    • TabError
      On parser encountering an indentation syntax error
  • SystemError
    On non-fatal interpreter error - bug - report it
  • TypeError
    On passing inappropriate type to built-in operator or function.
  • ValueError
    On argument error not covered by TypeError or more precise.
    • UnicodeError
      On Unicode-related encoding or decoding error.

Standard methods & operators redefinition in classes

Standard methods & operators map to special methods '__method__' and thus can be redefined (mostly in in user-defined classes), e.g.:
class C:
def __init__(self, v): self.value = v
def __add__(self, r): return self.value + r

a = C(3) # sort of like calling C.__init__(a, 3)
a + 4 # is equivalent to a.__add__(4)

Special methods for any class
Method Description
__init__(self, args) Instance initialization (on construction)
__del__(self) Called on object demise (refcount becomes 0)
__repr__(self) repr() and `...` conversions
__str__(self) str() and print statement
__cmp__(self,other) Compares self to other and returns <0, 0, or >0. Implements >, <, == etc...
__lt__(self, other) Called for self < other comparisons. Can return anything, or can raise an exception.
__le__(self, other) Called for self <= other comparisons. Can return anything, or can raise an exception.
__gt__(self, other) Called for self > other comparisons. Can return anything, or can raise an exception.
__ge__(self, other) Called for self >= other comparisons. Can return anything, or can raise an exception.
__eq__(self, other) Called for self == other comparisons. Can return anything, or can raise an exception.
__ne__(self, other) Called for self != other (and self <> other) comparisons. Can return anything, or can raise an exception.
__hash__(self) Compute a 32 bit hash code; hash() and dictionary ops
__nonzero__(self) Returns 0 or 1 for truth value testing. when this method is not defined, __len__() is called if defined; otherwise all class instances are considered "true".
__getattr__(self,name) Called when attribute lookup doesn't find name. See also __getattribute__.
__getattribute__( self, name) Same as __getattr__ but always called whenever the attribute name is accessed.
__setattr__(self, name, value) Called when setting an attribute (inside, don't use " = value", use instead "self.__dict__[name] = value")
__delattr__(self, name) Called to delete attribute <name>.
__call__(self, *args, **kwargs) Called when an instance is called as function: obj(arg1, arg2, ...) is a shorthand for obj.__call__(arg1, arg2, ...).


See list in the operator module. Operator function names are provided with 2 variants, with or without leading & trailing '__' (e.g. __add__ or add).

Numeric operations special methods
Operator Special method
self + other __add__(self, other)
self - other __sub__(self, other)
self * other __mul__(self, other)
self / other __div__(self, other) or __truediv__(self,other) if __future__.division is active.
self // other __floordiv__(self, other)
self % other __mod__(self, other)
divmod(self,other) __divmod__(self, other)
self ** other __pow__(self, other)
self & other __and__(self, other)
self ^ other __xor__(self, other)
self | other __or__(self, other)
self << other __lshift__(self, other)
self >> other __rshift__(self, other)
nonzero(self) __nonzero__(self) (used in boolean testing)
-self __neg__(self)
+self __pos__(self)
abs(self) __abs__(self)
~self __invert__(self) (bitwise)
self += other __iadd__(self, other)
self -= other __isub__(self, other)
self *= other __imul__(self, other)
self /= other __idiv__(self, other) or __itruediv__(self,other) if __future__.division is in effect.
self //= other __ifloordiv__(self, other)
self %= other __imod__(self, other)
self **= other __ipow__(self, other)
self &= other __iand__(self, other)
self ^= other __ixor__(self, other)
self |= other __ior__(self, other)
self <<= other __ilshift__(self, other)
self >>= other __irshift__(self, other)

built-in function Special method
int(self) __int__(self)
long(self) __long__(self)
float(self) __float__(self)
complex(self) __complex__(self)
oct(self) __oct__(self)
hex(self) __hex__(self)
coerce(self, other) __coerce__(self, other)
Right-hand-side equivalents for all binary operators exist; they are called when class instance is on r-h-s of operator:
  • a + 3  calls __add__(a, 3)
  • 3 + a  calls __radd__(a, 3)

Special operations for containers
Operation Special method Notes
All sequences and maps :
len(self) __len__(self) length of object, >= 0. Length 0 == false
self[k] __getitem__(self, k) Get element at indice /key k (indice starts at 0). Or, if k is a slice object, return a slice.
self[k] = value __setitem__(self, k, value) Set element at indice/key/slice k.
del self[k] __delitem__(self, k) Delete element at indice/key/slice k.
elt in self
elt not in self
__contains__(self, elt)
not __contains__(self, elt)
More efficient than std iteration thru sequence.
iter(self) __iter__(self) Returns an iterator on elements (keys for mappings <=> self.iterkeys()). See iterators.
Sequences, general methods, plus:
self[i:j] __getslice__(self, i, j) Deprecated since 2.0, replaced by __getitem__ with a slice object as parameter.
self[i:j] = seq __setslice__(self, i, j,seq) Deprecated since 2.0, replaced by __setitem__ with a slice object as parameter.
del self[i:j] __delslice__(self, i, j) Same as self[i:j] = [] - Deprecated since 2.0, replaced by __delitem__ with a slice object as parameter.
self * n __repeat__(self, n)  
self + other __concat__(self, other)  
Mappings, general methods, plus:
hash(self) __hash__(self) hashed value of object self is used for dictionary keys

Special informative state attributes for some types:

Tip: use module inspect to inspect live objects.

Lists & Dictionaries
Attribute Meaning
__methods__ (list, R/O): list of method names of the object Deprecated, use dir() instead
Attribute Meaning
__doc__ (string/None, R/O): doc string (<=> __dict__['__doc__'])
__name__ (string, R/O): module name (also in __dict__['__name__'])
__dict__ (dict, R/O): module's name space
__file__ (string/undefined, R/O): pathname of .pyc, .pyo or .pyd (undef for modules statically linked to the interpreter)
__path__ (list/undefined, R/W): List of directory paths where to find the package (for packages only).
Attribute Meaning
__doc__ (string/None, R/W): doc string (<=> __dict__['__doc__'])
__name__ (string, R/W): class name (also in __dict__['__name__'])
__bases__ (tuple, R/W): parent classes
__dict__ (dict, R/W): attributes (class name space)
Attribute Meaning
__class__ (class, R/W): instance's class
__dict__ (dict, R/W): attributes
User defined functions
Attribute Meaning
__doc__ (string/None, R/W): doc string
__name__ (string, R/O): function name
func_doc (R/W): same as __doc__
func_name (R/O): same as __name__
func_defaults (tuple/None, R/W): default args values if any
func_code (code, R/W): code object representing the compiled function body
func_globals (dict, R/O): ref to dictionary of func global variables
User-defined Methods
Attribute Meaning
__doc__ (string/None, R/O): doc string
__name__ (string, R/O): method name (same as im_func.__name__)
im_class (class, R/O): class defining the method (may be a base class)
im_self (instance/None, R/O): target instance object (None if unbound)
im_func (function, R/O): function object
Built-in Functions & methods
Attribute Meaning
__doc__ (string/None, R/O): doc string
__name__ (string, R/O): function name
__self__ [methods only] target object
__members__ list of attr names: ['__doc__','__name__','__self__']) Deprecated, use dir() instead
Attribute Meaning
co_name (string, R/O): function name
co_argcount (int, R/0): number of positional args
co_nlocals (int, R/O): number of local vars (including args)
co_varnames (tuple, R/O): names of local vars (starting with args)
co_code (string, R/O): sequence of bytecode instructions
co_consts (tuple, R/O): literals used by the bytecode, 1st one is function doc (or None)
co_names (tuple, R/O): names used by the bytecode
co_filename (string, R/O): filename from which the code was compiled
co_firstlineno (int, R/O): first line number of the function
co_lnotab (string, R/O): string encoding bytecode offsets to line numbers.
co_stacksize (int, R/O): required stack size (including local vars)
co_flags (int, R/O): flags for the interpreter bit 2 set if fct uses "*arg" syntax, bit 3 set if fct uses '**keywords' syntax
Attribute Meaning
f_back (frame/None, R/O): previous stack frame (toward the caller)
f_code (code, R/O): code object being executed in this frame
f_locals (dict, R/O): local vars
f_globals (dict, R/O): global vars
f_builtins (dict, R/O): built-in (intrinsic) names
f_restricted (int, R/O): flag indicating whether fct is executed in restricted mode
f_lineno (int, R/O): current line number
f_lasti (int, R/O): precise instruction (index into bytecode)
f_trace (function/None, R/W): debug hook called at start of each source line
f_exc_type (Type/None, R/W): Most recent exception type
f_exc_value (any, R/W): Most recent exception value
f_exc_traceback (traceback/None, R/W): Most recent exception traceback
Attribute Meaning
tb_next (frame/None, R/O): next level in stack trace (toward the frame where the exception occurred)
tb_frame (frame, R/O): execution frame of the current level
tb_lineno (int, R/O): line number where the exception occured
tb_lasti (int, R/O): precise instruction (index into bytecode)
Attribute Meaning
start (any/None, R/O): lowerbound, included
stop (any/None, R/O): upperbound, excluded
step (any/None, R/O): step value
Complex numbers
Attribute Meaning
real (float, R/O): real part
imag (float, R/O): imaginary part
Attribute Meaning
tolist (Built-in method, R/O): ?

Important Modules


System-specific parameters and functions. [Full doc]

Some sys variables
Variable Content
argv The list of command line arguments passed to a Python script. sys.argv[0] is the script name.
builtin_module_names A list of strings giving the names of all modules written in C that are linked into this interpreter.
byteorder Native byte order, either 'big'(-endian) or 'little'(-endian).
check_interval How often to check for thread switches or signals (measured in number of virtual machine instructions)
copyright A string containing the copyright pertaining to the Python interpreter.
Root directory where platform-dependent Python files are installed, e.g. 'C:\\Python23', '/usr'.
executable Name of executable binary of the Python interpreter (e.g. 'C:\\Python23\\python.exe', '/usr/bin/python')
exitfunc User can set to a parameterless function. It will get called before interpreter exits.
last_type, last_value, last_traceback Set only when an exception not handled and interpreter prints an error. Used by debuggers.
maxint Maximum positive value for integers. Since 2.2 integers and long integers are unified, thus integers have no limit.
maxunicode Largest supported code point for a Unicode character.
modules Dictionary of modules that have already been loaded.
path Search path for external modules. Can be modified by program. sys.path[0] == directory of script currently executed.
platform The current platform, e.g. "sunos5", "win32"
ps1, ps2 Prompts to use in interactive mode, normally ">>>" and "..."
stdin, stdout, stderr File objects used for I/O. One can redirect by assigning a new file object to them (or any object: with a method write(string) for stdout/stderr, or with a method readline() for stdin). __stdin__,__stdout__ and __stderr__ are the default values.
version String containing version info about Python interpreter.
version_info Tuple containing Python version info - (major, minor, micro, level, serial).
winver Version number used to form registry keys on Windows platforms (e.g. '2.2').
Some sys functions
Function Result
displayhook The function used to display the output of commands issued in interactive mode - defaults to the builtin repr(). __displayhook__ is the original value.
excepthook Can be set to a user defined function, to which any uncaught exceptions are passed.__excepthook__ is the original value.
exit(n) Exits with status n (usually 0 means OK). Raises SystemExit exception (hence can be caught and ignored by program)
getrefcount(object) Returns the reference count of the object. Generally 1 higher than you might expect, because of object arg temp reference.
setcheckinterval(interval) Sets the interpreter's thread switching interval (in number of bytecode instructions, default: 10 until 2.2, 100 from 2.3).
settrace(func) Sets a trace function: called before each line of code is exited.
setprofile(func) Sets a profile function for performance profiling.
exc_info() Info on exception currently being handled; this is a tuple (exc_type, exc_value, exc_traceback). Warning: assigning the traceback return value to a local variable in a function handling an exception will cause a circular reference.
setdefaultencoding(encoding) Change default Unicode encoding - defaults to 7-bit ASCII.
getrecursionlimit() Retrieve maximum recursion depth.
setrecursionlimit() Set maximum recursion depth (default 1000).


Miscellaneous operating system interfaces. [Full doc]

"synonym" for whatever OS-specific module (nt, mac, posix...) is proper for current environment. This module uses posix whenever possible.
(see also M.A. Lemburg's utility (now included in 2.3+)

Some os variables
Variable Meaning
name name of O/S-specific module (e.g. "posix", "mac", "nt")
path O/S-specific module for path manipulations.
On Unix, os.path.split() <=> posixpath.split()
curdir string used to represent current directory (eg '.')
pardir string used to represent parent directory (eg '..')
sep string used to separate directories ('/' or '\'). Tip: Use os.path.join() to build portable paths.
altsep Alternate separator if applicable (None otherwise)
pathsep character used to separate search path components (as in $PATH), eg. ';' for windows.
linesep line separator as used in text files, ie '\n' on Unix, '\r\n' on Dos/Win, '\r' on Mac.
Some os functions
Function Result
makedirs(path[, mode=0777]) Recursive directory creation (create required intermediary dirs); os.error if fails.
removedirs(path) Recursive directory delete (delete intermediary empty dirs); fails (os.error) if the directories are not empty.
renames(old, new) Recursive directory or file renaming; os.error if fails.


Posix OS interfaces. [Full doc]
Do not import this module directly, import os instead ! (see also module: shutil for file copy & remove functions)

posix Variables
Variable Meaning
environ dictionary of environment variables, e.g. posix.environ['HOME'].
error exception raised on POSIX-related error.
Corresponding value is tuple of errno code and perror() string.
Some posix functions
Function Result
chdir(path) Changes current directory to path.
chmod(path, mode) Changes the mode of path to the numeric mode
close(fd) Closes file descriptor fd opened with
_exit(n) Immediate exit, with no cleanups, no SystemExit, etc... Should use this to exit a child process.
execv(p, args) "Become" executable p with args args
getcwd() Returns a string representing the current working directory.
getcwdu() Returns a Unicode string representing the current working directory.
getpid() Returns the current process id.
fork() Like C's fork(). Returns 0 to child, child pid to parent [Not on Windows].
kill(pid, signal) Like C's kill [Not on Windows].
listdir(path) Lists (base)names of entries in directory path, excluding '.' and '..'. If path is a Unicode string, so will be the returned strings.
lseek(fd, pos, how) Sets current position in file fd to position pos, expressed as an offset relative to beginning of file (how=0), to current position (how=1), or to end of file (how=2).
mkdir(path[, mode]) Creates a directory named path with numeric mode (default 0777).
open(file, flags, mode) Like C's open(). Returns file descriptor. Use file object functions rather than this low level ones.
pipe() Creates a pipe. Returns pair of file descriptors (r, w) [Not on Windows].
popen(command, mode='r', bufSize=0) Opens a pipe to or from command. Result is a file object to read to or write from, as indicated by mode being 'r' or 'w'. Use it to catch a command output ('r' mode), or to feed it ('w' mode).
remove(path) See unlink.
rename(old, new) Renames/moves the file or directory old to new. [error if target name already exists]
renames(old, new) Recursive directory or file renaming function. Works like rename(), except creation of any intermediate directories needed to make the new pathname good is attempted first. After the rename, directories corresponding to rightmost path segments of the old name will be pruned away using removedirs().
rmdir(path) Removes the empty directory path
read(fd, n) Reads n bytes from file descriptor fd and return as string.
stat(path) Returns st_mode, st_ino, st_dev, st_nlink, st_uid,st_gid, st_size, st_atime, st_mtime, st_ctime. [st_ino, st_uid, st_gid are dummy on Windows]
system(command) Executes string command in a subshell. Returns exit status of subshell (usually 0 means OK).
times() Returns accumulated CPU times in sec (user, system, children's user, children's sys, elapsed real time) [3 last not on Windows].
unlink(path) Unlinks ("deletes") the file (not dir!) path. Same as: remove.
utime(path, (aTime, mTime)) Sets the access & modified time of the file to the given tuple of values.
wait() Waits for child process completion. Returns tuple of pid, exit_status [Not on Windows].
waitpid(pid, options) Waits for process pid to complete. Returns tuple of pid, exit_status [Not on Windows].
write(fd, str) Writes str to file fd. Returns nb of bytes written.


Posix pathname operations.
Do not import this module directly, import os instead and refer to this module as os.path. (e.g. os.path.exists(p))!

Some posixpath functions
Function Result
abspath(p) Returns absolute path for path p, taking current working dir in account.
commonprefix(list) Returns the longuest path prefix (taken character-by-character) that is a prefix of all paths in list (or '' if list empty).
dirname/basename(p) directory and name parts of the path p. See also split.
exists(p) True if string p is an existing path (file or directory).
expanduser(p) Returns string that is (a copy of) p with "~" expansion done.
expandvars(p) Returns string that is (a copy of) p with environment vars expanded. [Windows: case significant; must use Unix: $var notation, not %var%]
getmtime(filepath) Returns last modification time of filepath (integer nb of seconds since epoch).
getatime(filepath) Returns last access time of filepath (integer nb of seconds since epoch).
getsize(filepath) Returns the size in bytes of filepath. os.error if file inexistent or inaccessible.
isabs(p) True if string p is an absolute path.
isdir(p) True if string p is a directory.
islink(p) True if string p is a symbolic link.
ismount(p) True if string p is a mount point [true for all dirs on Windows].
join(p[,q[,...]]) Joins one or more path components intelligently.
split(p) Splits p into (head, tail) where tail is last pathname component and head is everything leading up to that. <=> (dirname(p), basename(p))
splitdrive(p) Splits path p in a pair ('drive:', tail) [Windows]
splitext(p) Splits into (root, ext) where last comp of root contains no periods and ext is empty or starts with a period.
walk(p, visit, arg) Calls the function visit with arguments (arg, dirname, names) for each directory recursively in the directory tree rooted at p (including p itself if it's a dir). The argument dirname specifies the visited directory, the argument names lists the files in the directory. The visit function may modify names to influence the set of directories visited below dirname, e.g. to avoid visiting certain parts of the tree.


High-level file operations (copying, deleting). [Full doc]

Main shutil functions
Function Result
copy(src, dest) Copies the contents of file src to file dest, retaining file permissions.
copytree(src, dest[, symlinks]) Recursively copies an entire directory tree rooted at src into dest (which should not already exist). If symlinks is true, links in src are kept as such in dest.
move(src, dest) Recursively moves a file or directory to a new location.
rmtree(path[, ignore_errors[, onerror]]) Deletes an entire directory tree, ignoring errors if ignore_errors is true, or calling onerror(func, path, sys.exc_info()) if supplied, with arguments func (faulty function), and path (concerned file).
(and also: copyfile, copymode, copystat, copy2)


Time access and conversions. [Full doc]

Variable Meaning
altzone Signed offset of local DST timezone in sec west of the 0th meridian.
daylight Non zero if a DST timezone is specified.
Some functions
Function Result
time() Returns a float representing UTC time in seconds since the epoch.
gmtime(secs), localtime(secs) Returns a tuple representing time : (year aaaa, month(1-12), day(1-31), hour(0-23), minute(0-59), second(0-59), weekday(0-6, 0 is monday), Julian day(1-366), daylight flag(-1,0 or 1)).
asctime(timeTuple), 24-character string of the following form: 'Sun Jun 20 23:21:05 1993'.
strftime(format, timeTuple) Returns a formated string representing time. See format in table below.
mktime(tuple) Inverse of localtime(). Returns a float.
strptime(string[, format]) Parses a formated string representing time, return tuple as in gmtime().
sleep(secs) Suspends execution for secs seconds. secs can be a float.
and also: clock, ctime.

Formatting in strftime()
Directive Meaning
%a Locale's abbreviated weekday name.
%A Locale's full weekday name.
%b Locale's abbreviated month name.
%B Locale's full month name.
%c Locale's appropriate date and time representation.
%d Day of the month as a decimal number [01,31].
%H Hour (24-hour clock) as a decimal number [00,23].
%I Hour (12-hour clock) as a decimal number [01,12].
%j Day of the year as a decimal number [001,366].
%m Month as a decimal number [01,12].
%M Minute as a decimal number [00,59].
%p Locale's equivalent of either AM or PM.
%S Second as a decimal number [00,61]. Yes, 61 !
%U Week number of the year (Sunday as the first day of the week) as a decimal number [00,53]. All days in a new year preceding the first Sunday are considered to be in week 0.
%w Weekday as a decimal number [0(Sunday),6].
%W Week number of the year (Monday as the first day of the week) as a decimal number [00,53]. All days in a new year preceding the first Sunday are considered to be in week 0.
%x Locale's appropriate date representation.
%X Locale's appropriate time representation.
%y Year without century as a decimal number [00,99].
%Y Year with century as a decimal number.
%Z Time zone name (or by no characters if no time zone exists).
%% A literal "%" character.


Common string operations. [Full doc]
As of Python 2.0, much (though not all) of the functionality provided by the string module have been superseded by built-in string methods - see Operations on strings for details.

Some string variables
Variable Meaning
digits The string '0123456789'.
hexdigits, octdigits Legal hexadecimal & octal digits.
letters, uppercase, lowercase, whitespace Strings containing the appropriate characters.
ascii_letters, ascii_lowercase, ascii_uppercase Same, taking the current locale in account.
index_error Exception raised by index() if substring not found.
Some string functions
Function Result
expandtabs(s, tabSize) Returns a copy of string s with tabs expanded.
find/rfind(s, sub[, start=0[, end=0]) Returns the lowest/highest index in s where the substring sub is found such that sub is wholly contained in s[start:end]. Return -1 if sub not found.
ljust/rjust/center(s, width) Returns a copy of string s; left/right justified/centered in a field of given width, padded with spaces. s is never truncated.
lower/upper(s) Returns a string that is (a copy of) s in lowercase/uppercase.
split(s[, sep=whitespace[, maxsplit=0]]) Returns a list containing the words of the string s, using the string sep as a separator.
join(words[, sep=' ']) Concatenates a list or tuple of words with intervening separators; inverse of split.
replace(s, old, new[, maxsplit=0] Returns a copy of string s with all occurrences of substring old replaced by new. Limits to maxsplit first substitutions if specified.
strip(s[, chars=None]) Returns a string that is (a copy of) s without leading and trailing chars (default: whitespace). Also: lstrip, rstrip.

re (sre)

Regular expression operations. [Full doc]

Handles Unicode strings. Implemented in new module sre, re now a mere front-end for compatibility.
Patterns are specified as strings. Tip: Use raw strings (e.g. r'\w*') to litteralize backslashes.

Regular expression syntax
Form Description
. Matches any character (including newline if DOTALL flag specified).
^ Matches start of the string (of every line in MULTILINE mode).
$ Matches end of the string (of every line in MULTILINE mode).
* 0 or more of preceding regular expression (as many as possible).
+ 1 or more of preceding regular expression (as many as possible).
? 0 or 1 occurrence of preceding regular expression.
*?, +?, ?? Same as *, + and ? but matches as few characters as possible.
{m,n} Matches from m to n repetitions of preceding RE.
{m,n}? Idem, attempting to match as few repetitions as possible.
[ ] Defines character set: e.g. '[a-zA-Z]' to match all letters (see also \w \S).
[^ ] Defines complemented character set: matches if char is NOT in set.
\ Escapes special chars '*?+&$|()' and introduces special sequences (see below). Due to Python string rules, write as '\\' or r'\' in the pattern string.
\\ Matches a litteral '\'; due to Python string rules, write as '\\\\' in pattern string, or better using raw string: r'\\'.
| Specifies alternative: 'foo|bar' matches 'foo' or 'bar'.
(...) Matches any RE inside (), and delimits a group.
(?:...) Idem but doesn't delimit a group (non capturing parenthesis).
(?P<name>...) Matches any RE inside (), and delimits a named group, (e.g. r'(?Pid[a-zA-Z_]\w*)' defines a group named id).
(?P=name) Matches whatever text was matched by the earlier group named name.
(?=...) Matches if ... matches next, but doesn't consume any of the string e.g. 'Isaac (?=Asimov)' matches 'Isaac' only if followed by 'Asimov'.
(?!...) Matches if ... doesn't match next. Negative of (?=...).
(?<=...) Matches if the current position in the string is preceded by a match for ... that ends at the current position. This is called a positive lookbehind assertion.
(?<!...) Matches if the current position in the string is not preceded by a match for .... This is called a negative lookbehind assertion.
(?#...) A comment; ignored.
(?letters) letters is one or more of 'i','L', 'm', 's', 'x'. Sets the corresponding flags (re.I, re.L, re.M, re.S, re.X) for the entire RE.
Special sequences
Sequence Description
number Matches content of the group of the same number; groups are numbered starting from 1.
\A Matches only at the start of the string.
\b Empty str at beginning or end of word: '\bis\b' matches 'is', but not 'his'.
\B Empty str NOT at beginning or end of word.
\d Any decimal digit (<=> [0-9]).
\D Any non-decimal digit char (<=> [^O-9]).
\s Any whitespace char (<=> [ \t\n\r\f\v]).
\S Any non-whitespace char (<=> [^ \t\n\r\f\v]).
\w Any alphaNumeric char (depends on LOCALE flag).
\W Any non-alphaNumeric char (depends on LOCALE flag).
\Z Matches only at the end of the string.
Variable Meaning
error Exception when pattern string isn't a valid regexp.
Function Result
compile(pattern[,flags=0]) Compiles a RE pattern string into a regular expression object.
Flags (combinable by |):
I or IGNORECASE or (?i)
case insensitive matching
L or LOCALE or (?L)
make \w, \W, \b, \B dependent on the current locale
M or MULTILINE or (?m)
matches every new line and not only start/end of the whole string
S or DOTALL or (?s)
'.' matches ALL chars, including newline
X or VERBOSE or (?x)
Ignores whitespace outside character sets
escape(string) Returns (a copy of) string with all non-alphanumerics backslashed.
match(pattern, string[, flags]) If 0 or more chars at beginning of string matches the RE pattern string, returns a corresponding MatchObject instance, or None if no match.
search(pattern, string[, flags]) Scans thru string for a location matching pattern, returns a corresponding MatchObject instance, or None if no match.
split(pattern, string[, maxsplit=0]) Splits string by occurrences of pattern. If capturing () are used in pattern, then occurrences of patterns or subpatterns are also returned.
findall(pattern, string) Returns a list of non-overlapping matches in pattern, either a list of groups or a list of tuples if the pattern has more than 1 group.
sub(pattern, repl, string[, count=0]) Returns string obtained by replacing the (count first) leftmost non-overlapping occurrences of pattern (a string or a RE object) in string by repl; repl can be a string or a function called with a single MatchObj arg, which must return the replacement string.
subn(pattern, repl, string[, count=0]) Same as sub(), but returns a tuple (newString, numberOfSubsMade).

Regular Expression Objects

RE objects are returned by the compile function.

re object attributes
Attribute Descrition
flags Flags arg used when RE obj was compiled, or 0 if none provided.
groupindex Dictionary of {group name: group number} in pattern.
pattern Pattern string from which RE obj was compiled.
re object methods
Method Result
match(string[, pos][, endpos]) If zero or more characters at the beginning of string match this regular expression, return a corresponding MatchObject instance. Return None if the string does not match the pattern; note that this is different from a zero-length match.
The optional second parameter pos gives an index in the string where the search is to start; it defaults to 0. This is not completely equivalent to slicing the string; the '' pattern character matches at the real beginning of the string and at positions just after a newline, but not necessarily at the index where the search is to start.
The optional parameter endpos limits how far the string will be searched; it will be as if the string is endpos characters long, so only the characters from pos to endpos will be searched for a match.
search(string[, pos][, endpos]) Scans through string looking for a location where this regular expression produces a match, and returns a corresponding MatchObject instance. Returns None if no position in the string matches the pattern; note that this is different from finding a zero-length match at some point in the string.
The optional pos and endpos parameters have the same meaning as for the match() method.
split(string[, maxsplit=0]) Identical to the split() function, using the compiled pattern.
findall(string) Identical to the findall() function, using the compiled pattern.
sub(repl, string[, count=0]) Identical to the sub() function, using the compiled pattern.
subn(repl, string[, count=0]) Identical to the subn() function, using the compiled pattern.

Match Objects

Match objects are returned by the match & search functions.

Match object attributes
Attribute Description
pos Value of pos passed to search or match functions; index into string at which RE engine started search.
endpos Value of endpos passed to search or match functions; index into string beyond which RE engine won't go.
re RE object whose match or search fct produced this MatchObj instance
string String passed to match() or search().
Match object functions
Function Result
group([g1, g2, ...]) Returns one or more groups of the match. If one arg, result is a string; if multiple args, result is a tuple with one item per arg. If gi is 0, return value is entire matching string; if 1 <= gi <= 99, return string matching group #gi (or None if no such group); gi may also be a group name.
groups() Returns a tuple of all groups of the match; groups not participating to the match have a value of None. Returns a string instead of tuple if len(tuple)== 1.
start(group), end(group) Returns indices of start & end of substring matched by group (or None if group exists but didn't contribute to the match).
span(group) Returns the 2-tuple (start(group), end(group)); can be (None, None) if group didn't contibute to the match.


For intensive number crunching, see also Numerical Python and the Python and Scientific computing page. [Full doc]

Name Value
pi 3.1415926535897931
e 2.7182818284590451
Name Result
acos(x) Return the arc cosine (measured in radians) of x.
asin(x) Return the arc sine (measured in radians) of x.
atan(x) Return the arc tangent (measured in radians) of x.
atan2(x, y) Return the arc tangent (measured in radians) of y/x. Unlike atan(y/x), the signs of both x and y are considered.
ceil(x) Return the ceiling of x as a float. This is the smallest integral value >= x.
cos(x) Return the cosine of x (measured in radians).
cosh(x) Return the hyperbolic cosine of x.
degrees(x) Convert angle x from radians to degrees.
exp(x) Return e raised to the power of x.
fabs(x) Return the absolute value of the float x.
floor(x) Return the floor of x as a float. This is the largest integral value <= x.
fmod(x, y) Return fmod(x, y), according to platform C. x % y may differ.
frexp(x) Return the mantissa and exponent of x, as pair (m, e). m is a float and e is an int, such that x = m * 2.**e. If x is 0, m and e are both 0. Else 0.5 <= abs(m) < 1.0.
hypot(x, y) Return the Euclidean distance sqrt(x*x + y*y).
ldexp(x, i) x * (2**i)
log(x[, base]) Return the logarithm of x to the given base. If the base is not specified, returns the natural logarithm (base e) of x.
log10(x) Return the base 10 logarithm of x.
modf(x) Return the fractional and integer parts of x. Both results carry the sign of x. The integer part is returned as a float.
pow(x, y) Return x**y (x to the power of y). Note that for y=2, it is more efficient to use x*x.
radians(x) Convert angle x from degrees to radians.
sin(x) Return the sine (measured in radians) of x.
sinh(x) Return the hyperbolic sine of x.
sqrt(x) Return the square root of x.
tan(x) Return the tangent (measured in radians) of x.
tanh(x) Return the hyperbolic tangent of x.


Parser for command line options. [Full doc]

This was the standard parser until Python 2.3, now superseded by optparse.
[see also: Richard Gruet's simple parser (shameless self promotion)]

getopt(list, optstr)    -- Similar to C. <optstr> is option letters to look for. 
Put ':' after letter if option takes arg. E.g.
# invocation was "python -c hi -a arg1 arg2"
opts, args = getopt.getopt(sys.argv[1:], 'ab:c:')
# opts would be
[('-c', 'hi'), ('-a', '')]
# args would be
['arg1', 'arg2']

List of modules and packages in base distribution

Built-ins and content of python Lib directory. The subdirectory Lib/site-packages contains platform-specific packages and modules.
[Python NT distribution, may be slightly different in other distributions]

Standard library modules
Operation Result
aifc Stuff to parse AIFF-C and AIFF files.
anydbm Generic interface to all dbm clones. (dbhash, gdbm, dbm, dumbdbm).
asynchat A class supporting chat-style (command/response) protocols.
asyncore Basic infrastructure for asynchronous socket service clients and servers.
atexit Register functions to be called at exit of Python interpreter.
audiodev Classes for manipulating audio devices (currently only for Sun and SGI).
base64 Conversions to/from base64 transport encoding as per RFC-1521.
BaseHTTPServer HTTP server base class
Bastion "Bastionification" utility (control access to instance vars).
bdb A generic Python debugger base class.
bsddb (Optional) improved BSD database interface [package].
binhex Macintosh binhex compression/decompression.
bisect Bisection algorithms.
bz2 BZ2 compression.
calendar Calendar printing functions.
cgi Wraps the WWW Forms Common Gateway Interface (CGI).
CGIHTTPServer CGI-savvy HTTP Server.
cmd A generic class to build line-oriented command interpreters.
cmp Efficiently compare files, boolean outcome only.
cmpcache Same, but caches 'stat' results for speed.
code Utilities needed to emulate Python's interactive interpreter.
codecs Lookup existing Unicode encodings and register new ones.
codeop Utilities to compile possibly incomplete Python source code.
colorsys Conversion functions between RGB and other color systems.
commands Execute shell commands via os.popen [Unix only].
compileall Force "compilation" of all .py files in a directory.
ConfigParser Configuration file parser (much like windows .ini files).
Cookie HTTP state (cookies) management.
copy Generic shallow and deep copying operations.
copy_reg Helper to provide extensibility for modules pickle/cPickle.
csv Tools to read comma-separated files (of variations thereof).
datetime Improved date/time types (date, time, datetime, timedelta).
dbhash (g)dbm-compatible interface to bsdhash.hashopen.
difflib Tool for comparing sequences, and computing the changes required to convert one into another.
dircache Sorted list of files in a dir, using a cache.
dircmp Defines a class to build directory diff tools on.
dis Bytecode disassembler.
distutils Package installation system.
distutils.command.register Register module in the Python package index (PyPI). This command plugin adds the register command to distutil scripts.
doctest Unit testing framework based on running examples embedded in docstrings.
DocXMLRPCServer Creation of self-documenting XML-RPC servers, using pydoc to create HTML API doc on the fly.
dospath Common operations on DOS pathnames.
dumbdbm A dumb and slow but simple dbm clone.
dump Print python code that reconstructs a variable.
dummy_threading Helpers to make it easier to write code that use threads where supported, but still runs on Python versions without thread support. The dummy modules simply run the threads sequentially.
encodings New codecs: idna (IDNA strings), koi8_u (Ukranian), palmos (PalmOS 3.5), punycode (Punycode IDNA codec), string_escape (Python string escape codec: replaces non-printable chars w/ Python-style string escapes).
exceptions Class based built-in exception hierarchy.
filecmp File and directory comparison.
fileinput Helper class to quickly write a loop over all standard input files.
find Find files directory hierarchy matching a pattern.
fnmatch Filename matching with shell patterns.
formatter Generic output formatting.
fpformat General floating point formatting functions.
ftplib An FTP client class. Based on RFC 959.
gc Perform garbage collection, obtain GC debug stats, and tune GC parameters.
getopt Standard command line processing. See also optparse.
getpass Utilities to get a password and/or the current user name.
gettext Internationalization and localization support.
glob Filename "globbing" utility.
gopherlib Gopher protocol client interface.
grep 'grep' utilities.
gzip Read & write gzipped files.
heapq Heap queue (priority queue) helpers.
hmac HMAC (Keyed-Hashing for Message Authentication).
hotshot.stones Helper to run the pystone benchmark under the Hotshot profiler.
htmlentitydefs HTML character entity references.
htmllib HTML2 parsing utilities
HTMLParser Simple HTML and XHTML parser.
httplib HTTP1 client class.
idlelib (package) Support library for the IDLE development environment.
ihooks Hooks into the "import" mechanism.
imaplib IMAP4 client.Based on RFC 2060.
imghdr Recognizing image files based on their first few bytes.
imputil Provides a way of writing customized import hooks.
inspect Get information about live Python objects.
itertools Tools to work with iterators and lazy sequences.
keyword List of Python keywords.
knee A Python re-implementation of hierarchical module import.
linecache Cache lines from files.
linuxaudiodev Linux /dev/audio support. Replaced by ossaudiodev (Linux).
locale Support for number formatting using the current locale settings.
logging (package) Tools for structured logging in log4j style.
macpath Pathname (or related) operations for the Macintosh [Mac].
macurl2path Mac specific module for conversion between pathnames and URLs [Mac].
mailbox Classes to handle Unix style, MMDF style, and MH style mailboxes.
mailcap Mailcap file handling (RFC 1524).
markupbase Shared support for scanning document type declarations in HTML and XHTML.
mhlib MH (mailbox) interface.
mimetools Various tools used by MIME-reading or MIME-writing programs.
mimetypes Guess the MIME type of a file.
MimeWriter Generic MIME writer. Deprecated since release 2.3. Use the email package instead.
mimify Mimification and unmimification of mail messages.
mmap Interface to memory-mapped files - they behave like mutable strings.
modulefinder Tools to find what modules a given Python program uses, without actually running the program.
multifile A readline()-style interface to the parts of a multipart message.
mutex Mutual exclusion -- for use with module sched. See also std module threading, and glock.
netrc Parses and encapsulates the netrc file format.
nntplib An NNTP client class. Based on RFC 977.
ntpath Common operations on Windows pathnames.
nturl2path Convert a NT pathname to a file URL and vice versa.
olddifflib Old version of difflib (helpers for computing deltas between objects)?
optparse Improved command-line option parsing library (see also getopt).
os OS routines for Mac, DOS, NT, or Posix depending on what system we're on.
os2emxpath os.path support for OS/2 EMX.
packmail Create a self-unpacking shell archive.
pdb A Python debugger.
pickle Pickling (save and restore) of Python objects (a faster C implementation exists in built-in module: cPickle).
pickletools Tools to analyze and disassemble pickles.
pipes Conversion pipeline templates.
pkgutil Tools to extend the module search path for a given package.
platform Get info about the underlying platform.
poly Polynomials.
popen2 Spawn a command with pipes to its stdin, stdout, and optionally stderr.
poplib A POP3 client class.
posixfile Extended file operations available in POSIX.
posixpath Common operations on POSIX pathnames.
pprint Support to pretty-print lists, tuples, & dictionaries recursively.
pre Support for regular expressions (RE) - see re.
profile Class for profiling python code.
pstats Class for printing reports on profiled python code.
pty Pseudo terminal utilities.
py_compile Routine to "compile" a .py file to a .pyc file.
pyclbr Parse a Python file and retrieve classes and methods.
pydoc Generate Python documentation in HTML or text for interactive use.
pyexpat Interface to the Expat XML parser.
PyUnit Unit test framework inspired by JUnit. See unittest.
Queue A multi-producer, multi-consumer queue.
quopri Conversions to/from quoted-printable transport encoding as per RFC 1521.
rand Don't use unless you want compatibility with C's rand().
random Random variable generators.
re Regular Expressions.
readline GNU readline interface [Unix].
reconvert Convert old ("regex") regular expressions to new syntax ("re").
regex_syntax Flags for regex.set_syntax().
regexp Backward compatibility for module "regexp" using "regex".
regsub Regexp-based split and replace using the obsolete regex module.
repr Redo repr() but with limits on most sizes.
rexec Restricted execution facilities ("safe" exec, eval, etc).
rfc822 Parse RFC-8222 mail headers.
rlcompleter Word completion for GNU readline 2.0.
robotparser Parse robot.txt files, useful for web spiders.
sched A generally useful event scheduler class.
sets A Set datatype implementation based on dictionaries (see Sets).
sgmllib A parser for SGML, using the derived class as a static DTD.
shelve Manage shelves of pickled objects.
shlex Lexical analyzer class for simple shell-like syntaxes.
shutil Utility functions for copying files and directory trees.
SimpleHTTPServer Simple HTTP Server.
SimpleXMLRPCServer Simple XML-RPC Server
site Append module search paths for third-party packages to sys.path.
smtpd An RFC 2821 smtp proxy.
smtplib SMTP/ESMTP client class.
sndhdr Several routines that help recognizing sound.
socket Socket operations and some related functions. Now supports timeouts thru function settimeout(t). Also supports SSL on Windows.
SocketServer Generic socket server classes.
sre Support for regular expressions (RE). See re.
stat Constants/functions for interpreting results of os.
statcache Maintain a cache of stat() information on files.
statvfs Constants for interpreting statvfs struct as returned by os.statvfs() and os.fstatvfs() (if they exist).
string A collection of string operations (see Strings).
stringprep Normalization and manipulation of Unicode strings.
StringIO File-like objects that read/write a string buffer (a faster C implementation exists in built-in module: cStringIO).
sunau Stuff to parse Sun and NeXT audio files.
sunaudio Interpret sun audio headers.
symbol Non-terminal symbols of Python grammar (from "graminit.h").
symtable Interface to the compiler's internal symbol tables.
tabnanny Check Python source for ambiguous indentation.
tarfile Tools to read and create TAR archives.
telnetlib TELNET client class. Based on RFC 854.
tempfile Temporary files and filenames.
textwrap Tools to wrap paragraphs of text.
threading Proposed new threading module, emulating a subset of Java's threading model.
threading_api (doc of the threading module).
timeit Benchmark tool.
toaiff Convert "arbitrary" sound files to AIFF (Apple and SGI's audio format).
token Token constants (from "token.h").
tokenize Tokenizer for Python source.
traceback Extract, format and print information about Python stack traces.
trace Tools to trace execution of a function or program.
tty Terminal utilities [Unix].
turtle LogoMation-like turtle graphics.
types Define names for all type symbols in the std interpreter.
tzparse Parse a timezone specification.
unicodedata Interface to unicode properties.
unittest Python unit testing framework, based on Erich Gamma's JUnit and Kent Beck's.
urllib Open an arbitrary URL.
urllib2 An extensible library for opening URLs using a variety of protocols.
urlparse Parse (absolute and relative) URLs.
user Hook to allow user-specified customization code to run.
UserDict A wrapper to allow subclassing of built-in dict class (useless with new-style classes. Since Python 2.2, dict is subclassable).
UserList A wrapper to allow subclassing of built-in list class (useless with new-style classes. Since Python 2.2, list is subclassable)
UserString A wrapper to allow subclassing of built-in string class (useless with new-style classes. Since Python 2.2, str is subclassable).
util some useful functions that don't fit elsewhere !!
uu Implementation of the UUencode and UUdecode functions.
warnings Python part of the warnings subsystem. Issue warnings, and filter unwanted warnings.
wave Stuff to parse WAVE files.
weakref Weak reference support for Python. Also allows the creation of proxy objects.
webbrowser Platform independent URL launcher.
whatsound Several routines that help recognizing sound files.
whichdb Guess which db package to use to open a db file.
whrandom Wichmann-Hill random number generator (obsolete, use random instead).
xdrlib Implements (a subset of) Sun XDR (eXternal Data Representation).
xmllib A parser for XML, using the derived class as static DTD.
xml.dom Classes for processing XML using the DOM (Document Object Model). 2.3: New modules expatbuilder, minicompat, NodeFilter, xmlbuilder.
xml.sax Classes for processing XML using the SAX API.
xmlrpclib An XML-RPC client interface for Python.
xreadlines Provides a sequence-like object for reading a file line-by-line without reading the entire file into memory. Deprecated since release 2.3. Use for line in file instead.
zipfile Read & write PK zipped files.
zipimport ZIP archive importer.
zmod Demonstration of abstruse mathematical concepts.

Workspace exploration and idiom hints

dir(<module>)                        list functions, variables in <module>
dir() get object keys, defaults to local name space
if __name__ == '__main__': main() invoke main if running as script
map(None, lst1, lst2, ...) merge lists
b = a[:] create copy of seq structure
_ (underscore) in interactive mode, is last value printed

Python Mode for Emacs

Emacs goodies available here.
(The following has not been revised, probably not up to date - any contribution welcome -)

Type C-c ? when in python-mode for extensive help.
Primarily for entering new code:
TAB indent line appropriately
LFD insert newline, then indent
DEL reduce indentation, or delete single character
Primarily for reindenting existing code:
C-c : guess py-indent-offset from file content; change locally
C-u C-c : ditto, but change globally
C-c TAB reindent region to match its context
C-c < shift region left by py-indent-offset
C-c > shift region right by py-indent-offset
C-c C-b mark block of lines
M-C-h mark smallest enclosing def
C-u M-C-h mark smallest enclosing class
C-c # comment out region of code
C-u C-c # uncomment region of code
C-c C-p move to statement preceding point
C-c C-n move to statement following point
C-c C-u move up to start of current block
M-C-a move to start of def
C-u M-C-a move to start of class
M-C-e move to end of def
C-u M-C-e move to end of class
C-c C-c sends the entire buffer to the Python interpreter
C-c | sends the current region
C-c ! starts a Python interpreter window; this will be used by
subsequent C-c C-c or C-c | commands
py-indent-offset indentation increment
py-block-comment-prefix comment string used by py-comment-region
py-python-command shell command to invoke Python interpreter
py-scroll-process-buffer t means always scroll Python process buffer
py-temp-directory directory used for temp files (if needed)
py-beep-if-tab-change ring the bell if tab-width is changed