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:mod:`csv` --- CSV File Reading and Writing
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===========================================
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:synopsis: Write and read tabular data to and from delimited files.
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.. sectionauthor:: Skip Montanaro <skip@pobox.com>
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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. There is no "CSV standard", so
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the format is operationally defined by the many applications which read and
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write it. The lack of a standard means that subtle differences often exist in
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the data produced and consumed by different applications. These differences can
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make it annoying to process CSV files from multiple sources. Still, while the
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delimiters and quoting characters vary, the overall format is similar enough
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that it is possible to write a single module which can efficiently manipulate
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such data, hiding the details of reading and writing the data from the
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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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This version of the :mod:`csv` module doesn't support Unicode input. Also,
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there are currently some issues regarding ASCII NUL characters. Accordingly,
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all input should be UTF-8 or printable ASCII to be safe; see the examples in
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section :ref:`csv-examples`. These restrictions will be removed in the future.
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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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The :mod:`csv` module defines the following functions:
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.. function:: reader(csvfile[, dialect='excel'][, fmtparam])
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Return a reader object which will iterate over lines in the given *csvfile*.
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*csvfile* can be any object which supports the :term:`iterator` protocol and returns a
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string each time its :meth:`next` method is called --- file objects and list
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objects are both suitable. If *csvfile* is a file object, it must be opened
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with the 'b' flag on platforms where that makes a difference. 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 *fmtparam* 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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All data read are returned as strings. No automatic data type conversion is
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The parser is quite strict with respect to multi-line quoted fields. Previously,
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if a line ended within a quoted field without a terminating newline character, a
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newline would be inserted into the returned field. This behavior caused problems
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when reading files which contained carriage return characters within fields.
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The behavior was changed to return the field without inserting newlines. As a
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consequence, if newlines embedded within fields are important, the input should
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be split into lines in a manner which preserves the newline characters.
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A short usage example::
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>>> spamReader = csv.reader(open('eggs.csv'), 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'][, fmtparam])
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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 must be opened with the
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'b' flag on platforms where that makes a difference. 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 *fmtparam* 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`. 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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>>> spamWriter = csv.writer(open('eggs.csv', 'w'), delimiter=' ',
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... quotechar='|', quoting=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][, fmtparam])
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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 *fmtparam* keyword arguments, or both, with keyword arguments overriding
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parameters of the dialect. For full details about the dialect 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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.. 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(csvfile[, fieldnames=None[, restkey=None[, restval=None[, dialect='excel'[, *args, **kwds]]]]])
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Create an object which operates like a regular reader but maps the information
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read into a dict whose keys are given by the optional *fieldnames* parameter.
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If the *fieldnames* parameter is omitted, the values in the first row of the
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*csvfile* will be used as the fieldnames. If the row read has fewer fields than
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the fieldnames sequence, the value of *restval* will be used as the default
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value. If the row read has more fields than the fieldnames sequence, the
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remaining data is added as a sequence keyed by the value of *restkey*. If the
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row read has fewer fields than the fieldnames sequence, the remaining keys take
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the value of the optional *restval* parameter. Any other optional or keyword
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arguments are passed to the underlying :class:`reader` instance.
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.. class:: DictWriter(csvfile, fieldnames[, restval=''[, extrasaction='raise'[, dialect='excel'[, *args, **kwds]]]])
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Create an object which operates like a regular writer but maps dictionaries onto
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output rows. The *fieldnames* parameter identifies the order in which values in
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the dictionary passed to the :meth:`writerow` method are written to the
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*csvfile*. The optional *restval* parameter specifies the value to be written
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if the dictionary is missing a key in *fieldnames*. If the dictionary passed to
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the :meth:`writerow` method contains a key not found in *fieldnames*, the
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optional *extrasaction* parameter indicates what action to take. If it is set
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to ``'raise'`` a :exc:`ValueError` is raised. If it is set to ``'ignore'``,
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extra values in the dictionary are ignored. Any other optional or keyword
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arguments are passed to the underlying :class:`writer` instance.
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Note that unlike the :class:`DictReader` class, the *fieldnames* parameter of
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the :class:`DictWriter` is not optional. Since Python's :class:`dict` objects
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are not ordered, there is not enough information available to deduce the order
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in which the row should be written to the *csvfile*.
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The :class:`Dialect` class is a container class relied on primarily for its
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attributes, which are used to define the parameters for a specific
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:class:`reader` or :class:`writer` instance.
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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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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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An example for :class:`Sniffer` use::
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csvfile = open("example.csv")
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dialect = csv.Sniffer().sniff(csvfile.read(1024))
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reader = csv.reader(csvfile, dialect)
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# ... process CSV file contents here ...
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The :mod:`csv` module defines the following constants:
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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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.. data:: QUOTE_NONNUMERIC
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Instructs :class:`writer` objects to quote all non-numeric fields.
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Instructs the reader to convert all non-quoted fields to type *float*.
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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` to perform no special processing of quote characters.
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The :mod:`csv` module defines the following exception:
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Raised by any of the functions when an error is detected.
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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 be
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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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.. attribute:: Dialect.lineterminator
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The string used to terminate lines produced by the :class:`writer`. It defaults
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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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.. 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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.. 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 :const:`QUOTE_\*` constants (see section
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:ref:`csv-contents`) and defaults to :const:`QUOTE_MINIMAL`.
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.. attribute:: Dialect.skipinitialspace
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When :const:`True`, whitespace immediately following the *delimiter* is ignored.
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The default is :const:`False`.
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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, parsed according
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to the current dialect.
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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:: csvreader.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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: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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a sequence 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 to
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.. method:: csvwriter.writerows(rows)
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Write all the *rows* parameters (a list of *row* objects as described above) to
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the writer's file object, formatted according to the current 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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The simplest example of reading a CSV file::
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reader = csv.reader(open("some.csv", "rb"))
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Reading a file with an alternate format::
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reader = csv.reader(open("passwd", "rb"), delimiter=':', quoting=csv.QUOTE_NONE)
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The corresponding simplest possible writing example is::
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writer = csv.writer(open("some.csv", "wb"))
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writer.writerows(someiterable)
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Registering a new dialect::
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csv.register_dialect('unixpwd', delimiter=':', quoting=csv.QUOTE_NONE)
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reader = csv.reader(open("passwd", "rb"), 'unixpwd')
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A slightly more advanced use of the reader --- catching and reporting errors::
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filename = "some.csv"
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reader = csv.reader(open(filename, "rb"))
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except csv.Error as e:
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sys.exit('file %s, line %d: %s' % (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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for row in csv.reader(['one,two,three']):
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The :mod:`csv` module doesn't directly support reading and writing Unicode, but
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it is 8-bit-clean save for some problems with ASCII NUL characters. So you can
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write functions or classes that handle the encoding and decoding for you as long
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as you avoid encodings like UTF-16 that use NULs. UTF-8 is recommended.
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:func:`unicode_csv_reader` below is a :term:`generator` that wraps :class:`csv.reader`
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to handle Unicode CSV data (a list of Unicode strings). :func:`utf_8_encoder`
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is a :term:`generator` that encodes the Unicode strings as UTF-8, one string (or row) at
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a time. The encoded strings are parsed by the CSV reader, and
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:func:`unicode_csv_reader` decodes the UTF-8-encoded cells back into Unicode::
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def unicode_csv_reader(unicode_csv_data, dialect=csv.excel, **kwargs):
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# csv.py doesn't do Unicode; encode temporarily as UTF-8:
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csv_reader = csv.reader(utf_8_encoder(unicode_csv_data),
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dialect=dialect, **kwargs)
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for row in csv_reader:
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# decode UTF-8 back to Unicode, cell by cell:
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yield [unicode(cell, 'utf-8') for cell in row]
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def utf_8_encoder(unicode_csv_data):
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for line in unicode_csv_data:
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yield line.encode('utf-8')
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For all other encodings the following :class:`UnicodeReader` and
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:class:`UnicodeWriter` classes can be used. They take an additional *encoding*
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parameter in their constructor and make sure that the data passes the real
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reader or writer encoded as UTF-8::
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import csv, codecs, io
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Iterator that reads an encoded stream and reencodes the input to UTF-8
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def __init__(self, f, encoding):
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self.reader = codecs.getreader(encoding)(f)
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return next(self.reader).encode("utf-8")
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A CSV reader which will iterate over lines in the CSV file "f",
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which is encoded in the given encoding.
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def __init__(self, f, dialect=csv.excel, encoding="utf-8", **kwds):
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f = UTF8Recoder(f, encoding)
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self.reader = csv.reader(f, dialect=dialect, **kwds)
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row = next(self.reader)
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return [unicode(s, "utf-8") for s in row]
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A CSV writer which will write rows to CSV file "f",
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which is encoded in the given encoding.
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def __init__(self, f, dialect=csv.excel, encoding="utf-8", **kwds):
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# Redirect output to a queue
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self.queue = io.StringIO()
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self.writer = csv.writer(self.queue, dialect=dialect, **kwds)
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self.encoder = codecs.getincrementalencoder(encoding)()
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def writerow(self, row):
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self.writer.writerow([s.encode("utf-8") for s in row])
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# Fetch UTF-8 output from the queue ...
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data = self.queue.getvalue()
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data = data.decode("utf-8")
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# ... and reencode it into the target encoding
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data = self.encoder.encode(data)
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# write to the target stream
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self.stream.write(data)
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self.queue.truncate(0)
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def writerows(self, rows):
added
removed
Doc/library/csv.rst
