:mod:`!csv` --- CSV file reading and writing

.. module:: csv

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

**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::

where :file:`eggs.csv` contains:

.. code-block:: text

.. 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::

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

.. code-block:: text

.. 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::

where :file:`names.csv` contains:

.. code-block:: text

.. 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::

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

.. code-block:: text

.. 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::

.. 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 listed in the :attr:`~Sniffer.preferred` attribute

are preferred, in that 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.

The :class:`Sniffer` class has the following attribute:

.. attribute:: preferred

The list of the delimiters preferred for breaking ties,

in the order of preference.

It can be modified.

Its initial value is ``[',', '\t', ';', ' ', ':']``.

An example for :class:`Sniffer` use::

.. _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*.

If *doublequote* is :const:`False` and *escapechar* is set,

the *quotechar* is escaped instead of causing the field to be quoted.

.. 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:: 3.10

Previously the *escapechar* itself was not escaped,

which lost it on reading.

.. 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::

Reading a file with an alternate format::

The corresponding simplest possible writing example is::

Since :func:`open` is used to open a CSV file for reading, the file

will by default be decoded into Unicode using UTF-8. To decode a file

using a different encoding, use the ``encoding`` argument of open::

The same applies to writing in something other than the default

encoding: specify the encoding argument when opening the output file.

Registering a new dialect::

A slightly more advanced use of the reader --- catching and reporting errors::

And while the module doesn't directly support parsing strings, it can easily be

done::

.. rubric:: Footnotes

.. [1] If ``newline=''`` is not specified, newlines embedded inside quoted fields