mirror of https://github.com/python/cpython
618 lines
22 KiB
ReStructuredText
618 lines
22 KiB
ReStructuredText
:mod:`csv` --- CSV File Reading and Writing
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===========================================
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.. module:: csv
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:synopsis: Write and read tabular data to and from delimited files.
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.. sectionauthor:: Skip Montanaro <skip.montanaro@gmail.com>
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**Source code:** :source:`Lib/csv.py`
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.. index::
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single: csv
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pair: data; tabular
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--------------
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The so-called CSV (Comma Separated Values) format is the most common import and
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export format for spreadsheets and databases. CSV format was used for many
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years prior to attempts to describe the format in a standardized way in
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:rfc:`4180`. The lack of a well-defined standard means that subtle differences
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often exist in the data produced and consumed by different applications. These
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differences can make it annoying to process CSV files from multiple sources.
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Still, while the delimiters and quoting characters vary, the overall format is
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similar enough that it is possible to write a single module which can
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efficiently manipulate such data, hiding the details of reading and writing the
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data from the programmer.
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The :mod:`csv` module implements classes to read and write tabular data in CSV
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format. It allows programmers to say, "write this data in the format preferred
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by Excel," or "read data from this file which was generated by Excel," without
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knowing the precise details of the CSV format used by Excel. Programmers can
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also describe the CSV formats understood by other applications or define their
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own special-purpose CSV formats.
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The :mod:`csv` module's :class:`reader` and :class:`writer` objects read and
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write sequences. Programmers can also read and write data in dictionary form
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using the :class:`DictReader` and :class:`DictWriter` classes.
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.. seealso::
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:pep:`305` - CSV File API
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The Python Enhancement Proposal which proposed this addition to Python.
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.. _csv-contents:
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Module Contents
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---------------
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The :mod:`csv` module defines the following functions:
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.. index::
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single: universal newlines; csv.reader function
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.. function:: reader(csvfile, dialect='excel', **fmtparams)
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Return a :ref:`reader object <reader-objects>` that will process
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lines from the given *csvfile*. A csvfile must be an iterable of
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strings, each in the reader's defined csv format.
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A csvfile is most commonly a file-like object or list.
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If *csvfile* is a file object,
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it should be opened with ``newline=''``. [1]_ An optional
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*dialect* parameter can be given which is used to define a set of parameters
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specific to a particular CSV dialect. It may be an instance of a subclass of
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the :class:`Dialect` class or one of the strings returned by the
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:func:`list_dialects` function. The other optional *fmtparams* keyword arguments
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can be given to override individual formatting parameters in the current
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dialect. For full details about the dialect and formatting parameters, see
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section :ref:`csv-fmt-params`.
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Each row read from the csv file is returned as a list of strings. No
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automatic data type conversion is performed unless the ``QUOTE_NONNUMERIC`` format
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option is specified (in which case unquoted fields are transformed into floats).
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A short usage example::
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>>> import csv
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>>> with open('eggs.csv', newline='') as csvfile:
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... spamreader = csv.reader(csvfile, delimiter=' ', quotechar='|')
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... for row in spamreader:
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... print(', '.join(row))
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Spam, Spam, Spam, Spam, Spam, Baked Beans
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Spam, Lovely Spam, Wonderful Spam
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.. function:: writer(csvfile, dialect='excel', **fmtparams)
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Return a writer object responsible for converting the user's data into delimited
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strings on the given file-like object. *csvfile* can be any object with a
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:func:`write` method. If *csvfile* is a file object, it should be opened with
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``newline=''`` [1]_. An optional *dialect*
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parameter can be given which is used to define a set of parameters specific to a
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particular CSV dialect. It may be an instance of a subclass of the
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:class:`Dialect` class or one of the strings returned by the
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:func:`list_dialects` function. The other optional *fmtparams* keyword arguments
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can be given to override individual formatting parameters in the current
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dialect. For full details about dialects and formatting parameters, see
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the :ref:`csv-fmt-params` section. To make it
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as easy as possible to interface with modules which implement the DB API, the
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value :const:`None` is written as the empty string. While this isn't a
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reversible transformation, it makes it easier to dump SQL NULL data values to
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CSV files without preprocessing the data returned from a ``cursor.fetch*`` call.
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All other non-string data are stringified with :func:`str` before being written.
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A short usage example::
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import csv
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with open('eggs.csv', 'w', newline='') as csvfile:
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spamwriter = csv.writer(csvfile, delimiter=' ',
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quotechar='|', quoting=csv.QUOTE_MINIMAL)
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spamwriter.writerow(['Spam'] * 5 + ['Baked Beans'])
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spamwriter.writerow(['Spam', 'Lovely Spam', 'Wonderful Spam'])
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.. function:: register_dialect(name[, dialect[, **fmtparams]])
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Associate *dialect* with *name*. *name* must be a string. The
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dialect can be specified either by passing a sub-class of :class:`Dialect`, or
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by *fmtparams* keyword arguments, or both, with keyword arguments overriding
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parameters of the dialect. For full details about dialects and formatting
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parameters, see section :ref:`csv-fmt-params`.
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.. function:: unregister_dialect(name)
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Delete the dialect associated with *name* from the dialect registry. An
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:exc:`Error` is raised if *name* is not a registered dialect name.
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.. function:: get_dialect(name)
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Return the dialect associated with *name*. An :exc:`Error` is raised if
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*name* is not a registered dialect name. This function returns an immutable
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:class:`Dialect`.
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.. function:: list_dialects()
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Return the names of all registered dialects.
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.. function:: field_size_limit([new_limit])
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Returns the current maximum field size allowed by the parser. If *new_limit* is
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given, this becomes the new limit.
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The :mod:`csv` module defines the following classes:
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.. class:: DictReader(f, fieldnames=None, restkey=None, restval=None, \
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dialect='excel', *args, **kwds)
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Create an object that operates like a regular reader but maps the
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information in each row to a :class:`dict` whose keys are given by the
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optional *fieldnames* parameter.
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The *fieldnames* parameter is a :term:`sequence`. If *fieldnames* is
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omitted, the values in the first row of file *f* will be used as the
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fieldnames. Regardless of how the fieldnames are determined, the
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dictionary preserves their original ordering.
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If a row has more fields than fieldnames, the remaining data is put in a
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list and stored with the fieldname specified by *restkey* (which defaults
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to ``None``). If a non-blank row has fewer fields than fieldnames, the
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missing values are filled-in with the value of *restval* (which defaults
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to ``None``).
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All other optional or keyword arguments are passed to the underlying
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:class:`reader` instance.
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If the argument passed to *fieldnames* is an iterator, it will be coerced to a :class:`list`.
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.. versionchanged:: 3.6
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Returned rows are now of type :class:`OrderedDict`.
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.. versionchanged:: 3.8
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Returned rows are now of type :class:`dict`.
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A short usage example::
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>>> import csv
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>>> with open('names.csv', newline='') as csvfile:
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... reader = csv.DictReader(csvfile)
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... for row in reader:
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... print(row['first_name'], row['last_name'])
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...
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Eric Idle
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John Cleese
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>>> print(row)
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{'first_name': 'John', 'last_name': 'Cleese'}
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.. class:: DictWriter(f, fieldnames, restval='', extrasaction='raise', \
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dialect='excel', *args, **kwds)
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Create an object which operates like a regular writer but maps dictionaries
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onto output rows. The *fieldnames* parameter is a :mod:`sequence
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<collections.abc>` of keys that identify the order in which values in the
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dictionary passed to the :meth:`writerow` method are written to file
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*f*. The optional *restval* parameter specifies the value to be
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written if the dictionary is missing a key in *fieldnames*. If the
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dictionary passed to the :meth:`writerow` method contains a key not found in
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*fieldnames*, the optional *extrasaction* parameter indicates what action to
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take.
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If it is set to ``'raise'``, the default value, a :exc:`ValueError`
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is raised.
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If it is set to ``'ignore'``, extra values in the dictionary are ignored.
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Any other optional or keyword arguments are passed to the underlying
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:class:`writer` instance.
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Note that unlike the :class:`DictReader` class, the *fieldnames* parameter
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of the :class:`DictWriter` class is not optional.
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If the argument passed to *fieldnames* is an iterator, it will be coerced to a :class:`list`.
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A short usage example::
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import csv
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with open('names.csv', 'w', newline='') as csvfile:
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fieldnames = ['first_name', 'last_name']
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writer = csv.DictWriter(csvfile, fieldnames=fieldnames)
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writer.writeheader()
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writer.writerow({'first_name': 'Baked', 'last_name': 'Beans'})
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writer.writerow({'first_name': 'Lovely', 'last_name': 'Spam'})
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writer.writerow({'first_name': 'Wonderful', 'last_name': 'Spam'})
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.. class:: Dialect
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The :class:`Dialect` class is a container class whose attributes contain
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information for how to handle doublequotes, whitespace, delimiters, etc.
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Due to the lack of a strict CSV specification, different applications
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produce subtly different CSV data. :class:`Dialect` instances define how
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:class:`reader` and :class:`writer` instances behave.
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All available :class:`Dialect` names are returned by :func:`list_dialects`,
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and they can be registered with specific :class:`reader` and :class:`writer`
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classes through their initializer (``__init__``) functions like this::
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import csv
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with open('students.csv', 'w', newline='') as csvfile:
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writer = csv.writer(csvfile, dialect='unix')
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^^^^^^^^^^^^^^
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.. class:: excel()
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The :class:`excel` class defines the usual properties of an Excel-generated CSV
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file. It is registered with the dialect name ``'excel'``.
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.. class:: excel_tab()
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The :class:`excel_tab` class defines the usual properties of an Excel-generated
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TAB-delimited file. It is registered with the dialect name ``'excel-tab'``.
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.. class:: unix_dialect()
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The :class:`unix_dialect` class defines the usual properties of a CSV file
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generated on UNIX systems, i.e. using ``'\n'`` as line terminator and quoting
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all fields. It is registered with the dialect name ``'unix'``.
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.. versionadded:: 3.2
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.. class:: Sniffer()
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The :class:`Sniffer` class is used to deduce the format of a CSV file.
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The :class:`Sniffer` class provides two methods:
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.. method:: sniff(sample, delimiters=None)
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Analyze the given *sample* and return a :class:`Dialect` subclass
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reflecting the parameters found. If the optional *delimiters* parameter
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is given, it is interpreted as a string containing possible valid
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delimiter characters.
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.. method:: has_header(sample)
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Analyze the sample text (presumed to be in CSV format) and return
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:const:`True` if the first row appears to be a series of column headers.
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Inspecting each column, one of two key criteria will be considered to
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estimate if the sample contains a header:
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- the second through n-th rows contain numeric values
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- the second through n-th rows contain strings where at least one value's
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length differs from that of the putative header of that column.
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Twenty rows after the first row are sampled; if more than half of columns +
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rows meet the criteria, :const:`True` is returned.
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.. note::
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This method is a rough heuristic and may produce both false positives and
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negatives.
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An example for :class:`Sniffer` use::
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with open('example.csv', newline='') as csvfile:
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dialect = csv.Sniffer().sniff(csvfile.read(1024))
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csvfile.seek(0)
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reader = csv.reader(csvfile, dialect)
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# ... process CSV file contents here ...
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.. _csv-constants:
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The :mod:`csv` module defines the following constants:
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.. data:: QUOTE_ALL
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Instructs :class:`writer` objects to quote all fields.
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.. data:: QUOTE_MINIMAL
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Instructs :class:`writer` objects to only quote those fields which contain
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special characters such as *delimiter*, *quotechar* or any of the characters in
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*lineterminator*.
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.. data:: QUOTE_NONNUMERIC
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Instructs :class:`writer` objects to quote all non-numeric fields.
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Instructs :class:`reader` objects to convert all non-quoted fields to type *float*.
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.. data:: QUOTE_NONE
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Instructs :class:`writer` objects to never quote fields. When the current
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*delimiter* occurs in output data it is preceded by the current *escapechar*
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character. If *escapechar* is not set, the writer will raise :exc:`Error` if
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any characters that require escaping are encountered.
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Instructs :class:`reader` objects to perform no special processing of quote characters.
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.. data:: QUOTE_NOTNULL
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Instructs :class:`writer` objects to quote all fields which are not
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``None``. This is similar to :data:`QUOTE_ALL`, except that if a
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field value is ``None`` an empty (unquoted) string is written.
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Instructs :class:`reader` objects to interpret an empty (unquoted) field as None and
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to otherwise behave as :data:`QUOTE_ALL`.
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.. data:: QUOTE_STRINGS
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Instructs :class:`writer` objects to always place quotes around fields
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which are strings. This is similar to :data:`QUOTE_NONNUMERIC`, except that if a
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field value is ``None`` an empty (unquoted) string is written.
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Instructs :class:`reader` objects to interpret an empty (unquoted) string as ``None`` and
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to otherwise behave as :data:`QUOTE_NONNUMERIC`.
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The :mod:`csv` module defines the following exception:
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.. exception:: Error
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Raised by any of the functions when an error is detected.
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.. _csv-fmt-params:
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Dialects and Formatting Parameters
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----------------------------------
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To make it easier to specify the format of input and output records, specific
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formatting parameters are grouped together into dialects. A dialect is a
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subclass of the :class:`Dialect` class having a set of specific methods and a
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single :meth:`validate` method. When creating :class:`reader` or
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:class:`writer` objects, the programmer can specify a string or a subclass of
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the :class:`Dialect` class as the dialect parameter. In addition to, or instead
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of, the *dialect* parameter, the programmer can also specify individual
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formatting parameters, which have the same names as the attributes defined below
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for the :class:`Dialect` class.
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Dialects support the following attributes:
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.. attribute:: Dialect.delimiter
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A one-character string used to separate fields. It defaults to ``','``.
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.. attribute:: Dialect.doublequote
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Controls how instances of *quotechar* appearing inside a field should
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themselves be quoted. When :const:`True`, the character is doubled. When
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:const:`False`, the *escapechar* is used as a prefix to the *quotechar*. It
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defaults to :const:`True`.
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On output, if *doublequote* is :const:`False` and no *escapechar* is set,
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:exc:`Error` is raised if a *quotechar* is found in a field.
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.. attribute:: Dialect.escapechar
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A one-character string used by the writer to escape the *delimiter* if *quoting*
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is set to :const:`QUOTE_NONE` and the *quotechar* if *doublequote* is
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:const:`False`. On reading, the *escapechar* removes any special meaning from
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the following character. It defaults to :const:`None`, which disables escaping.
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.. versionchanged:: 3.11
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An empty *escapechar* is not allowed.
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.. attribute:: Dialect.lineterminator
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The string used to terminate lines produced by the :class:`writer`. It defaults
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to ``'\r\n'``.
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.. note::
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The :class:`reader` is hard-coded to recognise either ``'\r'`` or ``'\n'`` as
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end-of-line, and ignores *lineterminator*. This behavior may change in the
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future.
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.. attribute:: Dialect.quotechar
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A one-character string used to quote fields containing special characters, such
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as the *delimiter* or *quotechar*, or which contain new-line characters. It
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defaults to ``'"'``.
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.. versionchanged:: 3.11
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An empty *quotechar* is not allowed.
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.. attribute:: Dialect.quoting
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Controls when quotes should be generated by the writer and recognised by the
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reader. It can take on any of the :ref:`QUOTE_\* constants <csv-constants>`
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and defaults to :const:`QUOTE_MINIMAL`.
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.. attribute:: Dialect.skipinitialspace
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When :const:`True`, spaces immediately following the *delimiter* are ignored.
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The default is :const:`False`.
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.. attribute:: Dialect.strict
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When ``True``, raise exception :exc:`Error` on bad CSV input.
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The default is ``False``.
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.. _reader-objects:
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Reader Objects
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--------------
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Reader objects (:class:`DictReader` instances and objects returned by the
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:func:`reader` function) have the following public methods:
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.. method:: csvreader.__next__()
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Return the next row of the reader's iterable object as a list (if the object
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was returned from :func:`reader`) or a dict (if it is a :class:`DictReader`
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instance), parsed according to the current :class:`Dialect`. Usually you
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should call this as ``next(reader)``.
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Reader objects have the following public attributes:
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.. attribute:: csvreader.dialect
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A read-only description of the dialect in use by the parser.
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.. attribute:: csvreader.line_num
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The number of lines read from the source iterator. This is not the same as the
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number of records returned, as records can span multiple lines.
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DictReader objects have the following public attribute:
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.. attribute:: DictReader.fieldnames
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If not passed as a parameter when creating the object, this attribute is
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initialized upon first access or when the first record is read from the
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file.
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Writer Objects
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--------------
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:class:`Writer` objects (:class:`DictWriter` instances and objects returned by
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the :func:`writer` function) have the following public methods. A *row* must be
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an iterable of strings or numbers for :class:`Writer` objects and a dictionary
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mapping fieldnames to strings or numbers (by passing them through :func:`str`
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first) for :class:`DictWriter` objects. Note that complex numbers are written
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out surrounded by parens. This may cause some problems for other programs which
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read CSV files (assuming they support complex numbers at all).
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.. method:: csvwriter.writerow(row)
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Write the *row* parameter to the writer's file object, formatted according
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to the current :class:`Dialect`. Return the return value of the call to the
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*write* method of the underlying file object.
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.. versionchanged:: 3.5
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Added support of arbitrary iterables.
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.. method:: csvwriter.writerows(rows)
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Write all elements in *rows* (an iterable of *row* objects as described
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above) to the writer's file object, formatted according to the current
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dialect.
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Writer objects have the following public attribute:
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.. attribute:: csvwriter.dialect
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A read-only description of the dialect in use by the writer.
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DictWriter objects have the following public method:
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.. method:: DictWriter.writeheader()
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Write a row with the field names (as specified in the constructor) to
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the writer's file object, formatted according to the current dialect. Return
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the return value of the :meth:`csvwriter.writerow` call used internally.
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.. versionadded:: 3.2
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.. versionchanged:: 3.8
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:meth:`writeheader` now also returns the value returned by
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the :meth:`csvwriter.writerow` method it uses internally.
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.. _csv-examples:
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Examples
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--------
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The simplest example of reading a CSV file::
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import csv
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with open('some.csv', newline='') as f:
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reader = csv.reader(f)
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for row in reader:
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print(row)
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Reading a file with an alternate format::
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import csv
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with open('passwd', newline='') as f:
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reader = csv.reader(f, delimiter=':', quoting=csv.QUOTE_NONE)
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for row in reader:
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print(row)
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The corresponding simplest possible writing example is::
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import csv
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with open('some.csv', 'w', newline='') as f:
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writer = csv.writer(f)
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writer.writerows(someiterable)
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Since :func:`open` is used to open a CSV file for reading, the file
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will by default be decoded into unicode using the system default
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encoding (see :func:`locale.getencoding`). To decode a file
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using a different encoding, use the ``encoding`` argument of open::
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import csv
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with open('some.csv', newline='', encoding='utf-8') as f:
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reader = csv.reader(f)
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for row in reader:
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print(row)
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The same applies to writing in something other than the system default
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encoding: specify the encoding argument when opening the output file.
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Registering a new dialect::
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import csv
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csv.register_dialect('unixpwd', delimiter=':', quoting=csv.QUOTE_NONE)
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with open('passwd', newline='') as f:
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reader = csv.reader(f, 'unixpwd')
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A slightly more advanced use of the reader --- catching and reporting errors::
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import csv, sys
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filename = 'some.csv'
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with open(filename, newline='') as f:
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reader = csv.reader(f)
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try:
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for row in reader:
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print(row)
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except csv.Error as e:
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sys.exit('file {}, line {}: {}'.format(filename, reader.line_num, e))
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And while the module doesn't directly support parsing strings, it can easily be
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done::
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import csv
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for row in csv.reader(['one,two,three']):
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print(row)
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.. rubric:: Footnotes
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.. [1] If ``newline=''`` is not specified, newlines embedded inside quoted fields
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will not be interpreted correctly, and on platforms that use ``\r\n`` linendings
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on write an extra ``\r`` will be added. It should always be safe to specify
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``newline=''``, since the csv module does its own
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(:term:`universal <universal newlines>`) newline handling.
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