cpython/Doc/library/csv.rst

:mod:`csv` --- CSV File Reading and Writing
===========================================

.. 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
:rfc:`4180`.  The lack of a well-defined 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 :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.


.. _csv-contents:

Module Contents
---------------

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:

.. class:: DictReader(csvfile, fieldnames=None, restkey=None, restval=None, \
                      dialect='excel', *args, **kwds)

   Create an object which operates like a regular reader but maps the
   information read into a dict whose keys are given by the optional
   *fieldnames* parameter.  The *fieldnames* parameter is a :mod:`sequence
   <collections.abc>` whose elements are associated with the fields of the
   input data in order. These elements become the keys of the resulting
   dictionary.  If the *fieldnames* parameter is omitted, the values in the
   first row of the *csvfile* will be used as the fieldnames.  If the row read
   has more fields than the fieldnames sequence, the remaining data is added as
   a sequence keyed by the value of *restkey*.  If the row read has fewer
   fields than the fieldnames sequence, the remaining keys take the value of
   the optional *restval* parameter.  Any other optional or keyword arguments
   are passed to the underlying :class:`reader` instance.


.. class:: DictWriter(csvfile, fieldnames, restval='', extrasaction='raise', \
                      dialect='excel', *args, **kwds)

   Create an object which operates like a regular writer but maps dictionaries
   onto output rows.  The *fieldnames* parameter is a :mod:`sequence
   <collections.abc>` of keys that identify the order in which values in the
   dictionary passed to the :meth:`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 :meth:`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 :exc:`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 :class:`writer`
   instance.

   Note that unlike the :class:`DictReader` class, the *fieldnames* parameter
   of the :class:`DictWriter` is not optional.  Since Python's :class:`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*.


.. class:: Dialect

   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.


.. class:: excel()

   The :class:`excel` class defines the usual properties of an Excel-generated CSV
   file.  It is registered with the dialect name ``'excel'``.


.. class:: excel_tab()

   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'``.


.. class:: unix_dialect()

   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


.. class:: Sniffer()

   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') 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.

.. _csv-fmt-params:

Dialects and Formatting Parameters
----------------------------------

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 be
   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
--------------

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, 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.



Writer Objects
--------------

:class:`Writer` objects (:class:`DictWriter` instances and objects returned by
the :func:`writer` function) have the following public methods.  A *row* must be
a sequence 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.


.. method:: csvwriter.writerows(rows)

   Write all the *rows* parameters (a list 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).

   .. versionadded:: 3.2


.. _csv-examples:

Examples
--------

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)


.. rubric:: Footnotes

.. [1] If ``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.