The CPython interpreter scans the command line and the environment for various settings.
CPython implementation detail: Other implementations’ command line schemes may differ. See Alternate Implementations for further resources.
When invoking Python, you may specify any of these options:
python [-BdEiOQsStuUvVWxX3?] [-c command | -m module-name | script | - ] [args]
The most common use case is, of course, a simple invocation of a script:
python myscript.py
The interpreter interface resembles that of the UNIX shell, but provides some additional methods of invocation:
In non-interactive mode, the entire input is parsed before it is executed.
An interface option terminates the list of options consumed by the interpreter, all consecutive arguments will end up in sys.argv – note that the first element, subscript zero (sys.argv[0]), is a string reflecting the program’s source.
Execute the Python code in command. command can be one ore more statements separated by newlines, with significant leading whitespace as in normal module code.
If this option is given, the first element of sys.argv will be "-c" and the current directory will be added to the start of sys.path (allowing modules in that directory to be imported as top level modules).
Search sys.path for the named module and execute its contents as the __main__ module.
Since the argument is a module name, you must not give a file extension (.py). The module-name should be a valid Python module name, but the implementation may not always enforce this (e.g. it may allow you to use a name that includes a hyphen).
Note
This option cannot be used with built-in modules and extension modules written in C, since they do not have Python module files. However, it can still be used for precompiled modules, even if the original source file is not available.
If this option is given, the first element of sys.argv will be the full path to the module file. As with the -c option, the current directory will be added to the start of sys.path.
Many standard library modules contain code that is invoked on their execution as a script. An example is the timeit module:
python -mtimeit -s 'setup here' 'benchmarked code here'
python -mtimeit -h # for details
See also
runpy.run_module() The actual implementation of this feature.
PEP 338 – Executing modules as scripts
New in version 2.4.
Changed in version 2.5: The named module can now be located inside a package.
Read commands from standard input (sys.stdin). If standard input is a terminal, -i is implied.
If this option is given, the first element of sys.argv will be "-" and the current directory will be added to the start of sys.path.
Execute the Python code contained in script, which must be a filesystem path (absolute or relative) referring to either a Python file, a directory containing a __main__.py file, or a zipfile containing a __main__.py file.
If this option is given, the first element of sys.argv will be the script name as given on the command line.
If the script name refers directly to a Python file, the directory containing that file is added to the start of sys.path, and the file is executed as the __main__ module.
If the script name refers to a directory or zipfile, the script name is added to the start of sys.path and the __main__.py file in that location is executed as the __main__ module.
Changed in version 2.5: Directories and zipfiles containing a __main__.py file at the top level are now considered valid Python scripts.
If no interface option is given, -i is implied, sys.argv[0] is an empty string ("") and the current directory will be added to the start of sys.path.
See also
If given, Python won’t try to write .pyc or .pyo files on the import of source modules. See also PYTHONDONTWRITEBYTECODE.
New in version 2.6.
Ignore all PYTHON* environment variables, e.g. PYTHONPATH and PYTHONHOME, that might be set.
New in version 2.2.
When a script is passed as first argument or the -c option is used, enter interactive mode after executing the script or the command, even when sys.stdin does not appear to be a terminal. The PYTHONSTARTUP file is not read.
This can be useful to inspect global variables or a stack trace when a script raises an exception. See also PYTHONINSPECT.
Division control. The argument must be one of the following:
Don’t add user site directory to sys.path
New in version 2.6.
See also
PEP 370 – Per user site-packages directory
Force stdin, stdout and stderr to be totally unbuffered. On systems where it matters, also put stdin, stdout and stderr in binary mode.
Note that there is internal buffering in file.readlines() and File Objects (for line in sys.stdin) which is not influenced by this option. To work around this, you will want to use file.readline() inside a while 1: loop.
See also PYTHONUNBUFFERED.
Warning control. Python’s warning machinery by default prints warning messages to sys.stderr. A typical warning message has the following form:
file:line: category: message
By default, each warning is printed once for each source line where it occurs. This option controls how often warnings are printed.
Multiple -W options may be given; when a warning matches more than one option, the action for the last matching option is performed. Invalid -W options are ignored (though, a warning message is printed about invalid options when the first warning is issued).
Warnings can also be controlled from within a Python program using the warnings module.
The simplest form of argument is one of the following action strings (or a unique abbreviation) by themselves:
The full form of argument is:
action:message:category:module:line
Here, action is as explained above but only applies to messages that match the remaining fields. Empty fields match all values; trailing empty fields may be omitted. The message field matches the start of the warning message printed; this match is case-insensitive. The category field matches the warning category. This must be a class name; the match test whether the actual warning category of the message is a subclass of the specified warning category. The full class name must be given. The module field matches the (fully-qualified) module name; this match is case-sensitive. The line field matches the line number, where zero matches all line numbers and is thus equivalent to an omitted line number.
Skip the first line of the source, allowing use of non-Unix forms of #!cmd. This is intended for a DOS specific hack only.
Note
The line numbers in error messages will be off by one.
Warn about Python 3.x incompatibilities which cannot be fixed trivially by 2to3. Among these are:
Using these will emit a DeprecationWarning.
New in version 2.6.
Turns all string literals into unicodes globally. Do not be tempted to use this option as it will probably break your world. It also produces .pyc files with a different magic number than normal. Instead, you can enable unicode literals on a per-module basis by using:
from __future__ import unicode_literals
at the top of the file. See __future__ for details.
These environment variables influence Python’s behavior.
Change the location of the standard Python libraries. By default, the libraries are searched in prefix/lib/pythonversion and exec_prefix/lib/pythonversion, where prefix and exec_prefix are installation-dependent directories, both defaulting to /usr/local.
When PYTHONHOME is set to a single directory, its value replaces both prefix and exec_prefix. To specify different values for these, set PYTHONHOME to prefix:exec_prefix.
Augment the default search path for module files. The format is the same as the shell’s PATH: one or more directory pathnames separated by os.pathsep (e.g. colons on Unix or semicolons on Windows). Non-existent directories are silently ignored.
In addition to normal directories, individual PYTHONPATH entries may refer to zipfiles containing pure Python modules (in either source or compiled form). Extension modules cannot be imported from zipfiles.
The default search path is installation dependent, but generally begins with prefix/lib/pythonversion (see PYTHONHOME above). It is always appended to PYTHONPATH.
An additional directory will be inserted in the search path in front of PYTHONPATH as described above under Interface options. The search path can be manipulated from within a Python program as the variable sys.path.
If this is set to a non-empty string it is equivalent to specifying the -i option.
This variable can also be modified by Python code using os.environ to force inspect mode on program termination.
If this is set, Python won’t try to write .pyc or .pyo files on the import of source modules.
New in version 2.6.
Overrides the encoding used for stdin/stdout/stderr, in the syntax encodingname:errorhandler. The :errorhandler part is optional and has the same meaning as in str.encode().
New in version 2.6.
If this is set, Python won’t add the user site directory to sys.path
New in version 2.6.
See also
PEP 370 – Per user site-packages directory
Sets the base directory for the user site directory
New in version 2.6.
See also
PEP 370 – Per user site-packages directory
Setting these variables only has an effect in a debug build of Python, that is, if Python was configured with the --with-pydebug build option.
If set, Python will print threading debug info.
Changed in version 2.6: Previously, this variable was called THREADDEBUG.