.. module:: csv :synopsis: Write and read tabular data to and from delimited files.
.. sectionauthor:: Skip Montanaro <skip@pobox.com>
Source code: :source:`Lib/csv.py`
.. index:: single: csv pair: data; tabular
The so-called CSV (Comma Separated Values) format is the most common import and export format for spreadsheets and databases. CSV format was used for many years prior to attempts to describe the format in a standardized way in :mod:`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 :mod:`csv` module's :class:`reader` and :class:`writer` objects read and write sequences. Programmers can also read and write data in dictionary form using the :class:`DictReader` and :class:`DictWriter` classes.
.. seealso:: :pep:`305` - CSV File API The Python Enhancement Proposal which proposed this addition to Python.
The :mod:`csv` module defines the following functions:
.. index:: single: universal newlines; csv.reader function
.. function:: reader(csvfile, dialect='excel', **fmtparams)
Return a reader object which will iterate over lines in the given *csvfile*.
*csvfile* can be any object which supports the :term:`iterator` protocol and returns a
string each time its :meth:`!__next__` method is called --- :term:`file objects
<file object>` 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 :class:`Dialect` class or one of the strings returned by the
:func:`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 :ref:`csv-fmt-params`.
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
>>> with open('eggs.csv', newline='') as csvfile:
... spamreader = csv.reader(csvfile, delimiter=' ', quotechar='|')
... for row in spamreader:
... print(', '.join(row))
Spam, Spam, Spam, Spam, Spam, Baked Beans
Spam, Lovely Spam, Wonderful Spam
.. function:: writer(csvfile, dialect='excel', **fmtparams)
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
:func:`write` method. 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
:class:`Dialect` class or one of the strings returned by the
:func:`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 :ref:`csv-fmt-params`. To make it
as easy as possible to interface with modules which implement the DB API, the
value :const:`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 :func:`str` before being written.
A short usage example::
import csv
with open('eggs.csv', 'w', newline='') as csvfile:
spamwriter = csv.writer(csvfile, delimiter=' ',
quotechar='|', quoting=csv.QUOTE_MINIMAL)
spamwriter.writerow(['Spam'] * 5 + ['Baked Beans'])
spamwriter.writerow(['Spam', 'Lovely Spam', 'Wonderful Spam'])
.. function:: register_dialect(name[, dialect[, **fmtparams]]) Associate *dialect* with *name*. *name* must be a string. The dialect can be specified either by passing a sub-class of :class:`Dialect`, or by *fmtparams* keyword arguments, or both, with keyword arguments overriding parameters of the dialect. For full details about the dialect and formatting parameters, see section :ref:`csv-fmt-params`.
.. function:: unregister_dialect(name) Delete the dialect associated with *name* from the dialect registry. An :exc:`Error` is raised if *name* is not a registered dialect name.
.. function:: get_dialect(name) Return the dialect associated with *name*. An :exc:`Error` is raised if *name* is not a registered dialect name. This function returns an immutable :class:`Dialect`.
.. function:: list_dialects() Return the names of all registered dialects.
.. function:: field_size_limit([new_limit]) Returns the current maximum field size allowed by the parser. If *new_limit* is given, this becomes the new limit.
The :mod:`csv` module defines the following classes:
The :class:`Dialect` class is a container class relied on primarily for its attributes, which are used to define the parameters for a specific :class:`reader` or :class:`writer` instance.
The :class:`excel` class defines the usual properties of an Excel-generated CSV
file. It is registered with the dialect name 'excel'.
The :class:`excel_tab` class defines the usual properties of an Excel-generated
TAB-delimited file. It is registered with the dialect name 'excel-tab'.
The :class:`unix_dialect` class defines the usual properties of a CSV file
generated on UNIX systems, i.e. using '\n' as line terminator and quoting
all fields. It is registered with the dialect name 'unix'.
.. versionadded:: 3.2
The :class:`Sniffer` class is used to deduce the format of a CSV file.
The :class:`Sniffer` class provides two methods:
.. method:: sniff(sample, delimiters=None) Analyze the given *sample* and return a :class:`Dialect` subclass reflecting the parameters found. If the optional *delimiters* parameter is given, it is interpreted as a string containing possible valid delimiter characters.
.. method:: has_header(sample) Analyze the sample text (presumed to be in CSV format) and return :const:`True` if the first row appears to be a series of column headers.
An example for :class:`Sniffer` use:
with open('example.csv', newline='') as csvfile:
dialect = csv.Sniffer().sniff(csvfile.read(1024))
csvfile.seek(0)
reader = csv.reader(csvfile, dialect)
# ... process CSV file contents here ...
The :mod:`csv` module defines the following constants:
.. data:: QUOTE_ALL Instructs :class:`writer` objects to quote all fields.
.. data:: QUOTE_MINIMAL Instructs :class:`writer` objects to only quote those fields which contain special characters such as *delimiter*, *quotechar* or any of the characters in *lineterminator*.
.. data:: QUOTE_NONNUMERIC Instructs :class:`writer` objects to quote all non-numeric fields. Instructs the reader to convert all non-quoted fields to type *float*.
.. data:: QUOTE_NONE Instructs :class:`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 :exc:`Error` if any characters that require escaping are encountered. Instructs :class:`reader` to perform no special processing of quote characters.
The :mod:`csv` module defines the following exception:
.. exception:: Error Raised by any of the functions when an error is detected.
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 :class:`Dialect` class having a set of specific methods and a single :meth:`validate` method. When creating :class:`reader` or :class:`writer` objects, the programmer can specify a string or a subclass of the :class:`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 :class:`Dialect` class.
Dialects support the following attributes:
.. attribute:: Dialect.delimiter A one-character string used to separate fields. It defaults to ``','``.
.. attribute:: Dialect.doublequote Controls how instances of *quotechar* appearing inside a field should themselves be quoted. When :const:`True`, the character is doubled. When :const:`False`, the *escapechar* is used as a prefix to the *quotechar*. It defaults to :const:`True`. On output, if *doublequote* is :const:`False` and no *escapechar* is set, :exc:`Error` is raised if a *quotechar* is found in a field.
.. attribute:: Dialect.escapechar A one-character string used by the writer to escape the *delimiter* if *quoting* is set to :const:`QUOTE_NONE` and the *quotechar* if *doublequote* is :const:`False`. On reading, the *escapechar* removes any special meaning from the following character. It defaults to :const:`None`, which disables escaping.
.. attribute:: Dialect.lineterminator The string used to terminate lines produced by the :class:`writer`. It defaults to ``'\r\n'``. .. note:: The :class:`reader` is hard-coded to recognise either ``'\r'`` or ``'\n'`` as end-of-line, and ignores *lineterminator*. This behavior may change in the future.
.. attribute:: Dialect.quotechar A one-character string used to quote fields containing special characters, such as the *delimiter* or *quotechar*, or which contain new-line characters. It defaults to ``'"'``.
.. attribute:: Dialect.quoting Controls when quotes should be generated by the writer and recognised by the reader. It can take on any of the :const:`QUOTE_\*` constants (see section :ref:`csv-contents`) and defaults to :const:`QUOTE_MINIMAL`.
.. attribute:: Dialect.skipinitialspace When :const:`True`, whitespace immediately following the *delimiter* is ignored. The default is :const:`False`.
.. attribute:: Dialect.strict When ``True``, raise exception :exc:`Error` on bad CSV input. The default is ``False``.
Reader objects (:class:`DictReader` instances and objects returned by the :func:`reader` function) have the following public methods:
.. method:: csvreader.__next__() Return the next row of the reader's iterable object as a list (if the object was returned from :func:`reader`) or a dict (if it is a :class:`DictReader` instance), parsed according to the current dialect. Usually you should call this as ``next(reader)``.
Reader objects have the following public attributes:
.. attribute:: csvreader.dialect A read-only description of the dialect in use by the parser.
.. attribute:: csvreader.line_num The number of lines read from the source iterator. This is not the same as the number of records returned, as records can span multiple lines.
DictReader objects have the following public attribute:
.. attribute:: csvreader.fieldnames If not passed as a parameter when creating the object, this attribute is initialized upon first access or when the first record is read from the file.
:class:`Writer` objects (:class:`DictWriter` instances and objects returned by the :func:`writer` function) have the following public methods. A row must be an iterable of strings or numbers for :class:`Writer` objects and a dictionary mapping fieldnames to strings or numbers (by passing them through :func:`str` first) for :class:`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).
.. method:: csvwriter.writerow(row) Write the *row* parameter to the writer's file object, formatted according to the current dialect. Return the return value of the call to the *write* method of the underlying file object. .. versionchanged:: 3.5 Added support of arbitrary iterables.
.. method:: csvwriter.writerows(rows) Write all elements in *rows* (an iterable of *row* objects as described above) to the writer's file object, formatted according to the current dialect.
Writer objects have the following public attribute:
.. attribute:: csvwriter.dialect A read-only description of the dialect in use by the writer.
DictWriter objects have the following public method:
.. method:: DictWriter.writeheader() Write a row with the field names (as specified in the constructor) to the writer's file object, formatted according to the current dialect. Return the return value of the :meth:`csvwriter.writerow` call used internally. .. versionadded:: 3.2 .. versionchanged:: 3.8 :meth:`writeheader` now also returns the value returned by the :meth:`csvwriter.writerow` method it uses internally.
The simplest example of reading a CSV file:
import csv
with open('some.csv', newline='') as f:
reader = csv.reader(f)
for row in reader:
print(row)
Reading a file with an alternate format:
import csv
with open('passwd', newline='') as f:
reader = csv.reader(f, delimiter=':', quoting=csv.QUOTE_NONE)
for row in reader:
print(row)
The corresponding simplest possible writing example is:
import csv
with open('some.csv', 'w', newline='') as f:
writer = csv.writer(f)
writer.writerows(someiterable)
Since :func:`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 :func:`locale.getpreferredencoding`). To decode a file
using a different encoding, use the encoding argument of open:
import csv
with open('some.csv', newline='', encoding='utf-8') as f:
reader = csv.reader(f)
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)
with open('passwd', newline='') as f:
reader = csv.reader(f, 'unixpwd')
A slightly more advanced use of the reader --- catching and reporting errors:
import csv, sys
filename = 'some.csv'
with open(filename, newline='') as f:
reader = csv.reader(f)
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
newline='' is not specified, newlines embedded inside quoted fields
will not be interpreted correctly, and on platforms that use \r\n linendings
on write an extra \r will be added. It should always be safe to specify
newline='', since the csv module does its own
(:term:`universal <universal newlines>`) newline handling.
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