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Doc/library/csv.rst
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Hugo van Kemenade
[3.14] gh-80678: Document the preferred delimiters of csv.Sniffer (GH-154336) (#154590)
31 июл 2026, 12:34
31 июл 2026, 12:34dff1325
:mod:`!csv` --- CSV File Reading and Writing
============================================

.. module:: csv
   :synopsis: Write and read tabular data to and from delimited files.

.. sectionauthor:: Skip Montanaro <skip.montanaro@gmail.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 :ref:`reader object <reader-objects>` that will process
   lines from the given *csvfile*.  A csvfile must be an iterable of
   strings, each in the reader's defined csv format.
   A csvfile is most commonly a file-like object or list.
   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 :data:`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

   where :file:`eggs.csv` contains:

   .. code-block:: text

      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
   :meth:`~io.TextIOBase.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 dialects and formatting parameters, see
   the :ref:`csv-fmt-params` section. 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'])

   which writes :file:`eggs.csv` containing:

   .. code-block:: text

      Spam Spam Spam Spam Spam |Baked Beans|
      Spam |Lovely Spam| |Wonderful Spam|


.. function:: register_dialect(name, /, dialect='excel', **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 dialects 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()
              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(f, fieldnames=None, restkey=None, restval=None, \
                      dialect='excel', *args, **kwds)

   Create an object that operates like a regular reader but maps the
   information in each row to a :class:`dict` whose keys are given by the
   optional *fieldnames* parameter.

   The *fieldnames* parameter is a :term:`sequence`.  If *fieldnames* is
   omitted, the values in the first row of file *f* will be used as the
   fieldnames and will be omitted from the results. If
   *fieldnames* is provided, they will be used and the first row will be
   included in the results.  Regardless of how the fieldnames are determined,
   the dictionary preserves their original ordering.

   If a row has more fields than fieldnames, the remaining data is put in a
   list and stored with the fieldname specified by *restkey* (which defaults
   to ``None``).  If a non-blank row has fewer fields than fieldnames, the
   missing values are filled-in with the value of *restval* (which defaults
   to ``None``).

   All other optional or keyword arguments are passed to the underlying
   :class:`reader` instance.

   If the argument passed to *fieldnames* is an iterator, it will be coerced to a :class:`list`.

   .. versionchanged:: 3.6
      Returned rows are now of type :class:`OrderedDict`.

   .. versionchanged:: 3.8
      Returned rows are now of type :class:`dict`.

   A short usage example::

       >>> import csv
       >>> with open('names.csv', newline='') as csvfile:
       ...     reader = csv.DictReader(csvfile)
       ...     for row in reader:
       ...         print(row['first_name'], row['last_name'])
       ...
       Eric Idle
       John Cleese

       >>> print(row)
       {'first_name': 'John', 'last_name': 'Cleese'}

   where :file:`names.csv` contains:

   .. code-block:: text

      first_name,last_name
      Eric,Idle
      John,Cleese


.. class:: DictWriter(f, 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:`~csvwriter.writerow` method are written to file
   *f*.  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:`~csvwriter.writerow` method contains a key not found in
   *fieldnames*, the optional *extrasaction* parameter indicates what action to
   take.
   If it is set to ``'raise'``, the default value, 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` class is not optional.

   If the argument passed to *fieldnames* is an iterator, it will be coerced to a :class:`list`.

   A short usage example::

       import csv

       with open('names.csv', 'w', newline='') as csvfile:
           fieldnames = ['first_name', 'last_name']
           writer = csv.DictWriter(csvfile, fieldnames=fieldnames)

           writer.writeheader()
           writer.writerow({'first_name': 'Baked', 'last_name': 'Beans'})
           writer.writerow({'first_name': 'Lovely', 'last_name': 'Spam'})
           writer.writerow({'first_name': 'Wonderful', 'last_name': 'Spam'})

   which writes :file:`names.csv` containing:

   .. code-block:: text

      first_name,last_name
      Baked,Beans
      Lovely,Spam
      Wonderful,Spam


.. class:: Dialect

   The :class:`Dialect` class is a container class whose attributes contain
   information for how to handle doublequotes, whitespace, delimiters, etc.
   Due to the lack of a strict CSV specification, different applications
   produce subtly different CSV data.  :class:`Dialect` instances define how
   :class:`reader` and :class:`writer` instances behave.

   All available :class:`Dialect` names are returned by :func:`list_dialects`,
   and they can be registered with specific :class:`reader` and :class:`writer`
   classes through their initializer (``__init__``) functions like this::

       import csv

       with open('students.csv', 'w', newline='') as csvfile:
           writer = csv.writer(csvfile, dialect='unix')


.. 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.

      If several delimiters fit the sample equally well ---
      for example if both ``','`` and ``';'`` split every row consistently ---
      the delimiters ``','``, ``'\t'``, ``';'``, ``' '`` and ``':'``
      are preferred, in this order,
      no matter how many times each of them occurs.

   .. 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.
      Inspecting each column, one of two key criteria will be considered to
      estimate if the sample contains a header:

      - the second through n-th rows contain numeric values
      - the second through n-th rows contain strings where at least one value's
        length differs from that of the putative header of that column.

      Twenty-one rows after the header are sampled; if more than half of the
      columns + rows meet the criteria, :const:`True` is returned.

   .. note::

      This method is a rough heuristic and may produce both false positives and
      negatives.

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 ...


.. _csv-constants:

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*, ``'\r'``, ``'\n'``
   or any of the characters in *lineterminator*.


.. data:: QUOTE_NONNUMERIC

   Instructs :class:`writer` objects to quote all non-numeric fields.

   Instructs :class:`reader` objects to convert all non-quoted fields to type :class:`float`.

   .. note::
      Some numeric types, such as :class:`bool`, :class:`~fractions.Fraction`,
      or :class:`~enum.IntEnum`, have a string representation that cannot be
      converted to :class:`float`.
      They cannot be read in the :data:`QUOTE_NONNUMERIC` and
      :data:`QUOTE_STRINGS` modes.

.. data:: QUOTE_NONE

   Instructs :class:`writer` objects to never quote fields.
   When the current *delimiter*, *quotechar*, *escapechar*, ``'\r'``, ``'\n'``
   or any of the characters in *lineterminator* 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.
   Set *quotechar* to ``None`` to prevent its escaping.

   Instructs :class:`reader` objects to perform no special processing of quote characters.

.. data:: QUOTE_NOTNULL

   Instructs :class:`writer` objects to quote all fields which are not
   ``None``.  This is similar to :data:`QUOTE_ALL`, except that if a
   field value is ``None`` an empty (unquoted) string is written.

   Instructs :class:`reader` objects to interpret an empty (unquoted) field
   as ``None`` and to otherwise behave as :data:`QUOTE_ALL`.

   .. versionadded:: 3.12

.. data:: QUOTE_STRINGS

   Instructs :class:`writer` objects to always place quotes around fields
   which are strings.  This is similar to :data:`QUOTE_NONNUMERIC`, except that if a
   field value is ``None`` an empty (unquoted) string is written.

   Instructs :class:`reader` objects to interpret an empty (unquoted) string as ``None`` and
   to otherwise behave as :data:`QUOTE_NONNUMERIC`.

   .. versionadded:: 3.12

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 containing various attributes
describing the format of the CSV file.  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 characters that
   require escaping:

      * the *delimiter*, the *quotechar*, ``'\r'``, ``'\n'`` and any of the
        characters in *lineterminator* are escaped if *quoting* is set to
        :const:`QUOTE_NONE`;
      * the *quotechar* is escaped if *doublequote* is :const:`False`;
      * the *escapechar* itself.

   On reading, the *escapechar* removes any special meaning from
   the following character. It defaults to :const:`None`, which disables escaping.

   .. versionchanged:: 3.11
      An empty *escapechar* is not allowed.

.. 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 the *quotechar*, or which contain new-line
   characters (``'\r'``, ``'\n'`` or any of the characters in *lineterminator*).
   It defaults to ``'"'``.
   Can be set to ``None`` to prevent escaping ``'"'`` if *quoting* is set
   to :const:`QUOTE_NONE`.

   .. versionchanged:: 3.11
      An empty *quotechar* is not allowed.

.. attribute:: Dialect.quoting

   Controls when quotes should be generated by the writer and recognised by the
   reader.  It can take on any of the :ref:`QUOTE_\* constants <csv-constants>`
   and defaults to :const:`QUOTE_MINIMAL` if *quotechar* is not ``None``,
   and :const:`QUOTE_NONE` otherwise.


.. attribute:: Dialect.skipinitialspace

   When :const:`True`, spaces immediately following the *delimiter* are ignored.
   The default is :const:`False`.  When combining ``delimiter=' '`` with
   ``skipinitialspace=True``, unquoted empty fields are not allowed.


.. attribute:: Dialect.strict

   When ``True``, raise exception :exc:`Error` on bad CSV input.
   The default is ``False``.

.. _reader-objects:

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 (if the object
   was returned from :func:`reader`) or a dict (if it is a :class:`DictReader`
   instance), parsed according to the current :class:`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:: DictReader.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
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 :class:`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.


.. _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.getencoding`).  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(f'file {filename}, line {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`` line endings
   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.