
``csv`` --- CSV File Reading and Writing
****************************************

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 ``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 ``csv`` module's ``reader`` and ``writer`` objects read and write
sequences.  Programmers can also read and write data in dictionary
form using the ``DictReader`` and ``DictWriter`` classes.

See also:

   **PEP 305** - CSV File API
      The Python Enhancement Proposal which proposed this addition to
      Python.


Module Contents
===============

The ``csv`` module defines the following functions:

csv.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
   *iterator* protocol and returns a string each time its
   ``__next__()`` method is called --- *file objects* 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 ``Dialect`` class or one of the strings returned by the
   ``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 *Dialects and
   Formatting Parameters*.

   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

csv.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 ``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 ``Dialect`` class or one of the
   strings returned by the ``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
   *Dialects and Formatting Parameters*. To make it as easy as
   possible to interface with modules which implement the DB API, the
   value ``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 ``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'])

csv.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
   ``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
   *Dialects and Formatting Parameters*.

csv.unregister_dialect(name)

   Delete the dialect associated with *name* from the dialect
   registry.  An ``Error`` is raised if *name* is not a registered
   dialect name.

csv.get_dialect(name)

   Return the dialect associated with *name*.  An ``Error`` is raised
   if *name* is not a registered dialect name.  This function returns
   an immutable ``Dialect``.

csv.list_dialects()

   Return the names of all registered dialects.

csv.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 ``csv`` module defines the following classes:

class class csv.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 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 ``reader`` instance.

class class csv.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 ``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 ``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 ``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 ``writer`` instance.

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

   The ``Dialect`` class is a container class relied on primarily for
   its attributes, which are used to define the parameters for a
   specific ``reader`` or ``writer`` instance.

class class csv.excel

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

class class csv.excel_tab

   The ``excel_tab`` class defines the usual properties of an Excel-
   generated TAB-delimited file.  It is registered with the dialect
   name ``'excel-tab'``.

class class csv.unix_dialect

   The ``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'``.

   New in version 3.2.

class class csv.Sniffer

   The ``Sniffer`` class is used to deduce the format of a CSV file.

   The ``Sniffer`` class provides two methods:

   sniff(sample, delimiters=None)

      Analyze the given *sample* and return a ``Dialect`` subclass
      reflecting the parameters found.  If the optional *delimiters*
      parameter is given, it is interpreted as a string containing
      possible valid delimiter characters.

   has_header(sample)

      Analyze the sample text (presumed to be in CSV format) and
      return ``True`` if the first row appears to be a series of
      column headers.

An example for ``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 ``csv`` module defines the following constants:

csv.QUOTE_ALL

   Instructs ``writer`` objects to quote all fields.

csv.QUOTE_MINIMAL

   Instructs ``writer`` objects to only quote those fields which
   contain special characters such as *delimiter*, *quotechar* or any
   of the characters in *lineterminator*.

csv.QUOTE_NONNUMERIC

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

   Instructs the reader to convert all non-quoted fields to type
   *float*.

csv.QUOTE_NONE

   Instructs ``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 ``Error`` if any characters that require escaping
   are encountered.

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

The ``csv`` module defines the following exception:

exception exception csv.Error

   Raised by any of the functions when an error is detected.


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 ``Dialect`` class having a set of
specific methods and a single ``validate()`` method.  When creating
``reader`` or ``writer`` objects, the programmer can specify a string
or a subclass of the ``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 ``Dialect`` class.

Dialects support the following attributes:

Dialect.delimiter

   A one-character string used to separate fields.  It defaults to
   ``','``.

Dialect.doublequote

   Controls how instances of *quotechar* appearing inside a field
   should be themselves be quoted.  When ``True``, the character is
   doubled. When ``False``, the *escapechar* is used as a prefix to
   the *quotechar*.  It defaults to ``True``.

   On output, if *doublequote* is ``False`` and no *escapechar* is
   set, ``Error`` is raised if a *quotechar* is found in a field.

Dialect.escapechar

   A one-character string used by the writer to escape the *delimiter*
   if *quoting* is set to ``QUOTE_NONE`` and the *quotechar* if
   *doublequote* is ``False``. On reading, the *escapechar* removes
   any special meaning from the following character. It defaults to
   ``None``, which disables escaping.

Dialect.lineterminator

   The string used to terminate lines produced by the ``writer``. It
   defaults to ``'\r\n'``.

   Note: The ``reader`` is hard-coded to recognise either ``'\r'`` or
     ``'\n'`` as end-of-line, and ignores *lineterminator*. This
     behavior may change in the future.

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 ``'"'``.

Dialect.quoting

   Controls when quotes should be generated by the writer and
   recognised by the reader.  It can take on any of the ``QUOTE_*``
   constants (see section *Module Contents*) and defaults to
   ``QUOTE_MINIMAL``.

Dialect.skipinitialspace

   When ``True``, whitespace immediately following the *delimiter* is
   ignored. The default is ``False``.

Dialect.strict

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


Reader Objects
==============

Reader objects (``DictReader`` instances and objects returned by the
``reader()`` function) have the following public methods:

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:

csvreader.dialect

   A read-only description of the dialect in use by the parser.

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:

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
==============

``Writer`` objects (``DictWriter`` instances and objects returned by
the ``writer()`` function) have the following public methods.  A *row*
must be a sequence of strings or numbers for ``Writer`` objects and a
dictionary mapping fieldnames to strings or numbers (by passing them
through ``str()`` first) for ``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).

csvwriter.writerow(row)

   Write the *row* parameter to the writer's file object, formatted
   according to the current dialect.

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:

csvwriter.dialect

   A read-only description of the dialect in use by the writer.

DictWriter objects have the following public method:

DictWriter.writeheader()

   Write a row with the field names (as specified in the constructor).

   New in version 3.2.


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 ``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 ``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 ]-

[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 (*universal*) newline handling.
