: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>

.. versionadded:: 2.3

.. 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.  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 :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.

.. note::

   This version of the :mod:`csv` module doesn't support Unicode input.  Also,

   there are currently some issues regarding ASCII NUL characters.  Accordingly,

   all input should be UTF-8 or printable ASCII to be safe; see the examples in

   section :ref:`csv-examples`. These restrictions will be removed in the future.

.. 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:

.. function:: reader(csvfile[, dialect='excel'][, fmtparam])

   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 --- file objects and list

   objects are both suitable.   If *csvfile* is a file object, it must be opened

   with the 'b' flag on platforms where that makes a difference.  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 *fmtparam* 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`.

   All data read are returned as strings.  No automatic data type conversion is

   performed.

   A short usage example::

      >>> import csv

      >>> spamReader = csv.reader(open('eggs.csv'), delimiter=' ', quotechar='|')

      >>> for row in spamReader:

      ...     print ', '.join(row)

      Spam, Spam, Spam, Spam, Spam, Baked Beans

      Spam, Lovely Spam, Wonderful Spam

   .. versionchanged:: 2.5

      The parser is now stricter with respect to multi-line quoted fields. Previously,

      if a line ended within a quoted field without a terminating newline character, a

      newline would be inserted into the returned field. This behavior caused problems

      when reading files which contained carriage return characters within fields.

      The behavior was changed to return the field without inserting newlines. As a

      consequence, if newlines embedded within fields are important, the input should

      be split into lines in a manner which preserves the newline characters.

.. function:: writer(csvfile[, dialect='excel'][, fmtparam])

   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 must be opened with the

   'b' flag on platforms where that makes a difference.  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 *fmtparam* 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

      >>> spamWriter = csv.writer(open('eggs.csv', 'w'), delimiter=' ',

      ...                         quotechar='|', quoting=QUOTE_MINIMAL)

      >>> spamWriter.writerow(['Spam'] * 5 + ['Baked Beans'])

      >>> spamWriter.writerow(['Spam', 'Lovely Spam', 'Wonderful Spam'])

.. function:: register_dialect(name[, dialect][, fmtparam])

   Associate *dialect* with *name*.  *name* must be a string or Unicode object. The

   dialect can be specified either by passing a sub-class of :class:`Dialect`, or

   by *fmtparam* 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.

   .. versionchanged:: 2.5

      This function now returns an immutable :class:`Dialect`.  Previously an

      instance of the requested dialect was returned.  Users could modify the

      underlying class, changing the behavior of active readers and writers.

.. 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.

   .. versionadded:: 2.5

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.

   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 fewer fields than

   the fieldnames sequence, the value of *restval* will be used as the default

   value.  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 identifies 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:: 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::

   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 :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`.

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.

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.

   .. versionadded:: 2.5

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.

   .. versionchanged:: 2.6

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.

.. _csv-examples:

Examples

--------

The simplest example of reading a CSV file::

   import csv

   reader = csv.reader(open("some.csv", "rb"))

   for row in reader:

       print row

Reading a file with an alternate format::

   import csv

   reader = csv.reader(open("passwd", "rb"), 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", "wb"))

   writer.writerows(someiterable)

Registering a new dialect::

   import csv

   csv.register_dialect('unixpwd', delimiter=':', quoting=csv.QUOTE_NONE)

   reader = csv.reader(open("passwd", "rb"), 'unixpwd')

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

   import csv, sys

   filename = "some.csv"

   reader = csv.reader(open(filename, "rb"))

   try:

       for row in reader:

           print row

   except csv.Error, e:

       sys.exit('file %s, line %d: %s' % (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

The :mod:`csv` module doesn't directly support reading and writing Unicode, but

it is 8-bit-clean save for some problems with ASCII NUL characters.  So you can

write functions or classes that handle the encoding and decoding for you as long

as you avoid encodings like UTF-16 that use NULs.  UTF-8 is recommended.

:func:`unicode_csv_reader` below is a :term:`generator` that wraps :class:`csv.reader`

to handle Unicode CSV data (a list of Unicode strings).  :func:`utf_8_encoder`

is a :term:`generator` that encodes the Unicode strings as UTF-8, one string (or row) at

a time.  The encoded strings are parsed by the CSV reader, and

:func:`unicode_csv_reader` decodes the UTF-8-encoded cells back into Unicode::

   import csv

   def unicode_csv_reader(unicode_csv_data, dialect=csv.excel, **kwargs):

       # csv.py doesn't do Unicode; encode temporarily as UTF-8:

       csv_reader = csv.reader(utf_8_encoder(unicode_csv_data),

                               dialect=dialect, **kwargs)

       for row in csv_reader:

           # decode UTF-8 back to Unicode, cell by cell:

           yield [unicode(cell, 'utf-8') for cell in row]

   def utf_8_encoder(unicode_csv_data):

       for line in unicode_csv_data:

           yield line.encode('utf-8')

For all other encodings the following :class:`UnicodeReader` and

:class:`UnicodeWriter` classes can be used. They take an additional *encoding*

parameter in their constructor and make sure that the data passes the real

reader or writer encoded as UTF-8::

   import csv, codecs, cStringIO

   class UTF8Recoder:

       """

       Iterator that reads an encoded stream and reencodes the input to UTF-8

       """

       def __init__(self, f, encoding):

           self.reader = codecs.getreader(encoding)(f)

       def __iter__(self):

           return self

       def next(self):

           return self.reader.next().encode("utf-8")

   class UnicodeReader:

       """

       A CSV reader which will iterate over lines in the CSV file "f",

       which is encoded in the given encoding.

       """