The so-called CSV (Comma Separated Values) format is the most common import and export format for spreadsheets and databases. There is no “CSV standard”, so the format is operationally defined by the many applications which read and write it. The lack of a standard means that subtle differences often exist in the data produced and consumed by different applications. These differences can make it annoying to process CSV files from multiple sources. Still, while the delimiters and quoting characters vary, the overall format is similar enough that it is possible to write a single module which can efficiently manipulate such data, hiding the details of reading and writing the data from the programmer.
The csv module implements classes to read and write tabular data in CSV format. It allows programmers to say, “write this data in the format preferred by Excel,” or “read data from this file which was generated by Excel,” without knowing the precise details of the CSV format used by Excel. Programmers can also describe the CSV formats understood by other applications or define their own special-purpose CSV formats.
The csv module’s reader and writer objects read and write sequences. Programmers can also read and write data in dictionary form using the DictReader and DictWriter classes.
See also
The csv module defines the following functions:
Return a reader object which will iterate over lines in the given csvfile. csvfile can be any object which supports the iterator protocol and returns a string each time its next() method is called — file objects and list objects are both suitable. If csvfile is a file object, it should be opened with newline=''. [1] An optional dialect parameter can be given which is used to define a set of parameters specific to a particular CSV dialect. It may be an instance of a subclass of the Dialect class or one of the strings returned by the list_dialects() function. The other optional fmtparams keyword arguments can be given to override individual formatting parameters in the current dialect. For full details about the dialect and formatting parameters, see section Dialects and Formatting Parameters.
Each row read from the csv file is returned as a list of strings. No automatic data type conversion is performed unless the QUOTE_NONNUMERIC format option is specified (in which case unquoted fields are transformed into floats).
A short usage example:
>>> import csv
>>> spamReader = csv.reader(open('eggs.csv', newline=''), delimiter=' ', quotechar='|')
>>> for row in spamReader:
... print(', '.join(row))
Spam, Spam, Spam, Spam, Spam, Baked Beans
Spam, Lovely Spam, Wonderful Spam
Return a writer object responsible for converting the user’s data into delimited strings on the given file-like object. csvfile can be any object with a write() method. An optional dialect parameter can be given which is used to define a set of parameters specific to a particular CSV dialect. It may be an instance of a subclass of the Dialect class or one of the strings returned by the list_dialects() function. The other optional fmtparams keyword arguments can be given to override individual formatting parameters in the current dialect. For full details about the dialect and formatting parameters, see section Dialects and Formatting Parameters. To make it as easy as possible to interface with modules which implement the DB API, the value None is written as the empty string. While this isn’t a reversible transformation, it makes it easier to dump SQL NULL data values to CSV files without preprocessing the data returned from a cursor.fetch* call. All other non-string data are stringified with str() before being written.
A short usage example:
>>> import csv
>>> spamWriter = csv.writer(open('eggs.csv', 'w'), delimiter=' ',
... quotechar='|', quoting=csv.QUOTE_MINIMAL)
>>> spamWriter.writerow(['Spam'] * 5 + ['Baked Beans'])
>>> spamWriter.writerow(['Spam', 'Lovely Spam', 'Wonderful Spam'])
The csv module defines the following classes:
Create an object which operates like a regular writer but maps dictionaries onto output rows. The fieldnames parameter identifies the order in which values in the dictionary passed to the writerow() method are written to the csvfile. The optional restval parameter specifies the value to be written if the dictionary is missing a key in fieldnames. If the dictionary passed to the writerow() method contains a key not found in fieldnames, the optional extrasaction parameter indicates what action to take. If it is set to 'raise' a ValueError is raised. If it is set to 'ignore', extra values in the dictionary are ignored. Any other optional or keyword arguments are passed to the underlying writer instance.
Note that unlike the DictReader class, the fieldnames parameter of the DictWriter is not optional. Since Python’s dict objects are not ordered, there is not enough information available to deduce the order in which the row should be written to the csvfile.
The Sniffer class is used to deduce the format of a CSV file.
The Sniffer class provides two methods:
An example for Sniffer use:
csvfile = open("example.csv")
dialect = csv.Sniffer().sniff(csvfile.read(1024))
csvfile.seek(0)
reader = csv.reader(csvfile, dialect)
# ... process CSV file contents here ...
The csv module defines the following constants:
Instructs writer objects to quote all non-numeric fields.
Instructs the reader to convert all non-quoted fields to type float.
Instructs writer objects to never quote fields. When the current delimiter occurs in output data it is preceded by the current escapechar character. If escapechar is not set, the writer will raise Error if any characters that require escaping are encountered.
Instructs reader to perform no special processing of quote characters.
The csv module defines the following exception:
To make it easier to specify the format of input and output records, specific formatting parameters are grouped together into dialects. A dialect is a subclass of the Dialect class having a set of specific methods and a single validate() method. When creating reader or writer objects, the programmer can specify a string or a subclass of the Dialect class as the dialect parameter. In addition to, or instead of, the dialect parameter, the programmer can also specify individual formatting parameters, which have the same names as the attributes defined below for the Dialect class.
Dialects support the following attributes:
Controls how instances of quotechar appearing inside a field should be themselves be quoted. When True, the character is doubled. When False, the escapechar is used as a prefix to the quotechar. It defaults to True.
On output, if doublequote is False and no escapechar is set, Error is raised if a quotechar is found in a field.
The string used to terminate lines produced by the writer. It defaults to '\r\n'.
Note
The reader is hard-coded to recognise either '\r' or '\n' as end-of-line, and ignores lineterminator. This behavior may change in the future.
Reader objects (DictReader instances and objects returned by the reader() function) have the following public methods:
Reader objects have the following public attributes:
DictReader objects have the following public attribute:
Writer objects (DictWriter instances and objects returned by the writer() function) have the following public methods. A row must be a sequence of strings or numbers for Writer objects and a dictionary mapping fieldnames to strings or numbers (by passing them through str() first) for DictWriter objects. Note that complex numbers are written out surrounded by parens. This may cause some problems for other programs which read CSV files (assuming they support complex numbers at all).
Writer objects have the following public attribute:
The simplest example of reading a CSV file:
import csv
reader = csv.reader(open("some.csv", newline=''))
for row in reader:
print(row)
Reading a file with an alternate format:
import csv
reader = csv.reader(open("passwd"), delimiter=':', quoting=csv.QUOTE_NONE)
for row in reader:
print(row)
The corresponding simplest possible writing example is:
import csv
writer = csv.writer(open("some.csv", "w"))
writer.writerows(someiterable)
Since open() is used to open a CSV file for reading, the file will by default be decoded into unicode using the system default encoding (see locale.getpreferredencoding()). To decode a file using a different encoding, use the encoding argument of open:
import csv
reader = csv.reader(open("some.csv", newline='', encoding='utf-8'))
for row in reader:
print(row)
The same applies to writing in something other than the system default encoding: specify the encoding argument when opening the output file.
Registering a new dialect:
import csv
csv.register_dialect('unixpwd', delimiter=':', quoting=csv.QUOTE_NONE)
reader = csv.reader(open("passwd"), 'unixpwd')
A slightly more advanced use of the reader — catching and reporting errors:
import csv, sys
filename = "some.csv"
reader = csv.reader(open(filename, newline=''))
try:
for row in reader:
print(row)
except csv.Error as e:
sys.exit('file {}, line {}: {}'.format(filename, reader.line_num, e))
And while the module doesn’t directly support parsing strings, it can easily be done:
import csv
for row in csv.reader(['one,two,three']):
print(row)
Footnotes
| [1] | If newline='' is not specified, newlines embedded inside quoted fields will not be interpreted correctly. It should always be safe to specify newline='', since the csv module does its own universal newline handling on input. |