cpython/Doc/library/codecs.rst

:mod:`codecs` --- Codec registry and base classes
=================================================

.. module:: codecs
   :synopsis: Encode and decode data and streams.

.. moduleauthor:: Marc-André Lemburg <mal@lemburg.com>
.. sectionauthor:: Marc-André Lemburg <mal@lemburg.com>
.. sectionauthor:: Martin v. Löwis <martin@v.loewis.de>

**Source code:** :source:`Lib/codecs.py`

.. index::
   single: Unicode
   single: Codecs
   pair: Codecs; encode
   pair: Codecs; decode
   single: streams
   pair: stackable; streams

--------------

This module defines base classes for standard Python codecs (encoders and
decoders) and provides access to the internal Python codec registry, which
manages the codec and error handling lookup process. Most standard codecs
are :term:`text encodings <text encoding>`, which encode text to bytes,
but there are also codecs provided that encode text to text, and bytes to
bytes. Custom codecs may encode and decode between arbitrary types, but some
module features are restricted to use specifically with
:term:`text encodings <text encoding>`, or with codecs that encode to
:class:`bytes`.

The module defines the following functions for encoding and decoding with
any codec:

.. function:: encode(obj, encoding='utf-8', errors='strict')

   Encodes *obj* using the codec registered for *encoding*.

   *Errors* may be given to set the desired error handling scheme. The
   default error handler is ``'strict'`` meaning that encoding errors raise
   :exc:`ValueError` (or a more codec specific subclass, such as
   :exc:`UnicodeEncodeError`). Refer to :ref:`codec-base-classes` for more
   information on codec error handling.

.. function:: decode(obj, encoding='utf-8', errors='strict')

   Decodes *obj* using the codec registered for *encoding*.

   *Errors* may be given to set the desired error handling scheme. The
   default error handler is ``'strict'`` meaning that decoding errors raise
   :exc:`ValueError` (or a more codec specific subclass, such as
   :exc:`UnicodeDecodeError`). Refer to :ref:`codec-base-classes` for more
   information on codec error handling.

The full details for each codec can also be looked up directly:

.. function:: lookup(encoding)

   Looks up the codec info in the Python codec registry and returns a
   :class:`CodecInfo` object as defined below.

   Encodings are first looked up in the registry's cache. If not found, the list of
   registered search functions is scanned. If no :class:`CodecInfo` object is
   found, a :exc:`LookupError` is raised. Otherwise, the :class:`CodecInfo` object
   is stored in the cache and returned to the caller.

.. class:: CodecInfo(encode, decode, streamreader=None, streamwriter=None, incrementalencoder=None, incrementaldecoder=None, name=None)

   Codec details when looking up the codec registry. The constructor
   arguments are stored in attributes of the same name:


   .. attribute:: name

      The name of the encoding.


   .. attribute:: encode
                  decode

      The stateless encoding and decoding functions. These must be
      functions or methods which have the same interface as
      the :meth:`~Codec.encode` and :meth:`~Codec.decode` methods of Codec
      instances (see :ref:`Codec Interface <codec-objects>`).
      The functions or methods are expected to work in a stateless mode.


   .. attribute:: incrementalencoder
                  incrementaldecoder

      Incremental encoder and decoder classes or factory functions.
      These have to provide the interface defined by the base classes
      :class:`IncrementalEncoder` and :class:`IncrementalDecoder`,
      respectively. Incremental codecs can maintain state.


   .. attribute:: streamwriter
                  streamreader

      Stream writer and reader classes or factory functions. These have to
      provide the interface defined by the base classes
      :class:`StreamWriter` and :class:`StreamReader`, respectively.
      Stream codecs can maintain state.

To simplify access to the various codec components, the module provides
these additional functions which use :func:`lookup` for the codec lookup:

.. function:: getencoder(encoding)

   Look up the codec for the given encoding and return its encoder function.

   Raises a :exc:`LookupError` in case the encoding cannot be found.


.. function:: getdecoder(encoding)

   Look up the codec for the given encoding and return its decoder function.

   Raises a :exc:`LookupError` in case the encoding cannot be found.


.. function:: getincrementalencoder(encoding)

   Look up the codec for the given encoding and return its incremental encoder
   class or factory function.

   Raises a :exc:`LookupError` in case the encoding cannot be found or the codec
   doesn't support an incremental encoder.


.. function:: getincrementaldecoder(encoding)

   Look up the codec for the given encoding and return its incremental decoder
   class or factory function.

   Raises a :exc:`LookupError` in case the encoding cannot be found or the codec
   doesn't support an incremental decoder.


.. function:: getreader(encoding)

   Look up the codec for the given encoding and return its :class:`StreamReader`
   class or factory function.

   Raises a :exc:`LookupError` in case the encoding cannot be found.


.. function:: getwriter(encoding)

   Look up the codec for the given encoding and return its :class:`StreamWriter`
   class or factory function.

   Raises a :exc:`LookupError` in case the encoding cannot be found.

Custom codecs are made available by registering a suitable codec search
function:

.. function:: register(search_function)

   Register a codec search function. Search functions are expected to take one
   argument, being the encoding name in all lower case letters, and return a
   :class:`CodecInfo` object. In case a search function cannot find
   a given encoding, it should return ``None``.

   .. note::

      Search function registration is not currently reversible,
      which may cause problems in some cases, such as unit testing or
      module reloading.

While the builtin :func:`open` and the associated :mod:`io` module are the
recommended approach for working with encoded text files, this module
provides additional utility functions and classes that allow the use of a
wider range of codecs when working with binary files:

.. function:: open(filename, mode='r', encoding=None, errors='strict', buffering=1)

   Open an encoded file using the given *mode* and return an instance of
   :class:`StreamReaderWriter`, providing transparent encoding/decoding.
   The default file mode is ``'r'``, meaning to open the file in read mode.

   .. note::

      Underlying encoded files are always opened in binary mode.
      No automatic conversion of ``'\n'`` is done on reading and writing.
      The *mode* argument may be any binary mode acceptable to the built-in
      :func:`open` function; the ``'b'`` is automatically added.

   *encoding* specifies the encoding which is to be used for the file.
   Any encoding that encodes to and decodes from bytes is allowed, and
   the data types supported by the file methods depend on the codec used.

   *errors* may be given to define the error handling. It defaults to ``'strict'``
   which causes a :exc:`ValueError` to be raised in case an encoding error occurs.

   *buffering* has the same meaning as for the built-in :func:`open` function.  It
   defaults to line buffered.


.. function:: EncodedFile(file, data_encoding, file_encoding=None, errors='strict')

   Return a :class:`StreamRecoder` instance, a wrapped version of *file*
   which provides transparent transcoding. The original file is closed
   when the wrapped version is closed.

   Data written to the wrapped file is decoded according to the given
   *data_encoding* and then written to the original file as bytes using
   *file_encoding*. Bytes read from the original file are decoded
   according to *file_encoding*, and the result is encoded
   using *data_encoding*.

   If *file_encoding* is not given, it defaults to *data_encoding*.

   *errors* may be given to define the error handling. It defaults to
   ``'strict'``, which causes :exc:`ValueError` to be raised in case an encoding
   error occurs.


.. function:: iterencode(iterator, encoding, errors='strict', **kwargs)

   Uses an incremental encoder to iteratively encode the input provided by
   *iterator*. This function is a :term:`generator`.
   The *errors* argument (as well as any
   other keyword argument) is passed through to the incremental encoder.

   This function requires that the codec accept text :class:`str` objects
   to encode. Therefore it does not support bytes-to-bytes encoders such as
   ``base64_codec``.


.. function:: iterdecode(iterator, encoding, errors='strict', **kwargs)

   Uses an incremental decoder to iteratively decode the input provided by
   *iterator*. This function is a :term:`generator`.
   The *errors* argument (as well as any
   other keyword argument) is passed through to the incremental decoder.

   This function requires that the codec accept :class:`bytes` objects
   to decode. Therefore it does not support text-to-text encoders such as
   ``rot_13``, although ``rot_13`` may be used equivalently with
   :func:`iterencode`.


The module also provides the following constants which are useful for reading
and writing to platform dependent files:


.. data:: BOM
          BOM_BE
          BOM_LE
          BOM_UTF8
          BOM_UTF16
          BOM_UTF16_BE
          BOM_UTF16_LE
          BOM_UTF32
          BOM_UTF32_BE
          BOM_UTF32_LE

   These constants define various byte sequences,
   being Unicode byte order marks (BOMs) for several encodings. They are
   used in UTF-16 and UTF-32 data streams to indicate the byte order used,
   and in UTF-8 as a Unicode signature. :const:`BOM_UTF16` is either
   :const:`BOM_UTF16_BE` or :const:`BOM_UTF16_LE` depending on the platform's
   native byte order, :const:`BOM` is an alias for :const:`BOM_UTF16`,
   :const:`BOM_LE` for :const:`BOM_UTF16_LE` and :const:`BOM_BE` for
   :const:`BOM_UTF16_BE`. The others represent the BOM in UTF-8 and UTF-32
   encodings.


.. _codec-base-classes:

Codec Base Classes
------------------

The :mod:`codecs` module defines a set of base classes which define the
interfaces for working with codec objects, and can also be used as the basis
for custom codec implementations.

Each codec has to define four interfaces to make it usable as codec in Python:
stateless encoder, stateless decoder, stream reader and stream writer. The
stream reader and writers typically reuse the stateless encoder/decoder to
implement the file protocols. Codec authors also need to define how the
codec will handle encoding and decoding errors.


.. _surrogateescape:
.. _error-handlers:

Error Handlers
^^^^^^^^^^^^^^

To simplify and standardize error handling,
codecs may implement different error handling schemes by
accepting the *errors* string argument.  The following string values are
defined and implemented by all standard Python codecs:

.. tabularcolumns:: |l|L|

+-------------------------+-----------------------------------------------+
| Value                   | Meaning                                       |
+=========================+===============================================+
| ``'strict'``            | Raise :exc:`UnicodeError` (or a subclass);    |
|                         | this is the default.  Implemented in          |
|                         | :func:`strict_errors`.                        |
+-------------------------+-----------------------------------------------+
| ``'ignore'``            | Ignore the malformed data and continue        |
|                         | without further notice.  Implemented in       |
|                         | :func:`ignore_errors`.                        |
+-------------------------+-----------------------------------------------+

The following error handlers are only applicable to
:term:`text encodings <text encoding>`:

.. index::
   single: ? (question mark); replacement character
   single: \ (backslash); escape sequence
   single: \x; escape sequence
   single: \u; escape sequence
   single: \U; escape sequence
   single: \N; escape sequence

+-------------------------+-----------------------------------------------+
| Value                   | Meaning                                       |
+=========================+===============================================+
| ``'replace'``           | Replace with a suitable replacement           |
|                         | marker; Python will use the official          |
|                         | ``U+FFFD`` REPLACEMENT CHARACTER for the      |
|                         | built-in codecs on decoding, and '?' on       |
|                         | encoding.  Implemented in                     |
|                         | :func:`replace_errors`.                       |
+-------------------------+-----------------------------------------------+
| ``'xmlcharrefreplace'`` | Replace with the appropriate XML character    |
|                         | reference (only for encoding).  Implemented   |
|                         | in :func:`xmlcharrefreplace_errors`.          |
+-------------------------+-----------------------------------------------+
| ``'backslashreplace'``  | Replace with backslashed escape sequences.    |
|                         | Implemented in                                |
|                         | :func:`backslashreplace_errors`.              |
+-------------------------+-----------------------------------------------+
| ``'namereplace'``       | Replace with ``\N{...}`` escape sequences     |
|                         | (only for encoding).  Implemented in          |
|                         | :func:`namereplace_errors`.                   |
+-------------------------+-----------------------------------------------+
| ``'surrogateescape'``   | On decoding, replace byte with individual     |
|                         | surrogate code ranging from ``U+DC80`` to     |
|                         | ``U+DCFF``.  This code will then be turned    |
|                         | back into the same byte when the              |
|                         | ``'surrogateescape'`` error handler is used   |
|                         | when encoding the data.  (See :pep:`383` for  |
|                         | more.)                                        |
+-------------------------+-----------------------------------------------+

In addition, the following error handler is specific to the given codecs:

+-------------------+------------------------+-------------------------------------------+
| Value             | Codecs                 | Meaning                                   |
+===================+========================+===========================================+
|``'surrogatepass'``| utf-8, utf-16, utf-32, | Allow encoding and decoding of surrogate  |
|                   | utf-16-be, utf-16-le,  | codes.  These codecs normally treat the   |
|                   | utf-32-be, utf-32-le   | presence of surrogates as an error.       |
+-------------------+------------------------+-------------------------------------------+

.. versionadded:: 3.1
   The ``'surrogateescape'`` and ``'surrogatepass'`` error handlers.

.. versionchanged:: 3.4
   The ``'surrogatepass'`` error handlers now works with utf-16\* and utf-32\* codecs.

.. versionadded:: 3.5
   The ``'namereplace'`` error handler.

.. versionchanged:: 3.5
   The ``'backslashreplace'`` error handlers now works with decoding and
   translating.

The set of allowed values can be extended by registering a new named error
handler:

.. function:: register_error(name, error_handler)

   Register the error handling function *error_handler* under the name *name*.
   The *error_handler* argument will be called during encoding and decoding
   in case of an error, when *name* is specified as the errors parameter.

   For encoding, *error_handler* will be called with a :exc:`UnicodeEncodeError`
   instance, which contains information about the location of the error. The
   error handler must either raise this or a different exception, or return a
   tuple with a replacement for the unencodable part of the input and a position
   where encoding should continue. The replacement may be either :class:`str` or
   :class:`bytes`.  If the replacement is bytes, the encoder will simply copy
   them into the output buffer. If the replacement is a string, the encoder will
   encode the replacement.  Encoding continues on original input at the
   specified position. Negative position values will be treated as being
   relative to the end of the input string. If the resulting position is out of
   bound an :exc:`IndexError` will be raised.

   Decoding and translating works similarly, except :exc:`UnicodeDecodeError` or
   :exc:`UnicodeTranslateError` will be passed to the handler and that the
   replacement from the error handler will be put into the output directly.


Previously registered error handlers (including the standard error handlers)
can be looked up by name:

.. function:: lookup_error(name)

   Return the error handler previously registered under the name *name*.

   Raises a :exc:`LookupError` in case the handler cannot be found.

The following standard error handlers are also made available as module level
functions:

.. function:: strict_errors(exception)

   Implements the ``'strict'`` error handling: each encoding or
   decoding error raises a :exc:`UnicodeError`.


.. function:: replace_errors(exception)

   Implements the ``'replace'`` error handling (for :term:`text encodings
   <text encoding>` only): substitutes ``'?'`` for encoding errors
   (to be encoded by the codec), and ``'\ufffd'`` (the Unicode replacement
   character) for decoding errors.


.. function:: ignore_errors(exception)

   Implements the ``'ignore'`` error handling: malformed data is ignored and
   encoding or decoding is continued without further notice.


.. function:: xmlcharrefreplace_errors(exception)

   Implements the ``'xmlcharrefreplace'`` error handling (for encoding with
   :term:`text encodings <text encoding>` only): the
   unencodable character is replaced by an appropriate XML character reference.


.. function:: backslashreplace_errors(exception)

   Implements the ``'backslashreplace'`` error handling (for
   :term:`text encodings <text encoding>` only): malformed data is
   replaced by a backslashed escape sequence.

.. function:: namereplace_errors(exception)

   Implements the ``'namereplace'`` error handling (for encoding with
   :term:`text encodings <text encoding>` only): the
   unencodable character is replaced by a ``\N{...}`` escape sequence.

   .. versionadded:: 3.5


.. _codec-objects:

Stateless Encoding and Decoding
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The base :class:`Codec` class defines these methods which also define the
function interfaces of the stateless encoder and decoder:


.. method:: Codec.encode(input[, errors])

   Encodes the object *input* and returns a tuple (output object, length consumed).
   For instance, :term:`text encoding` converts
   a string object to a bytes object using a particular
   character set encoding (e.g., ``cp1252`` or ``iso-8859-1``).

   The *errors* argument defines the error handling to apply.
   It defaults to ``'strict'`` handling.

   The method may not store state in the :class:`Codec` instance. Use
   :class:`StreamWriter` for codecs which have to keep state in order to make
   encoding efficient.

   The encoder must be able to handle zero length input and return an empty object
   of the output object type in this situation.


.. method:: Codec.decode(input[, errors])

   Decodes the object *input* and returns a tuple (output object, length
   consumed).  For instance, for a :term:`text encoding`, decoding converts
   a bytes object encoded using a particular
   character set encoding to a string object.

   For text encodings and bytes-to-bytes codecs,
   *input* must be a bytes object or one which provides the read-only
   buffer interface -- for example, buffer objects and memory mapped files.

   The *errors* argument defines the error handling to apply.
   It defaults to ``'strict'`` handling.

   The method may not store state in the :class:`Codec` instance. Use
   :class:`StreamReader` for codecs which have to keep state in order to make
   decoding efficient.

   The decoder must be able to handle zero length input and return an empty object
   of the output object type in this situation.


Incremental Encoding and Decoding
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The :class:`IncrementalEncoder` and :class:`IncrementalDecoder` classes provide
the basic interface for incremental encoding and decoding. Encoding/decoding the
input isn't done with one call to the stateless encoder/decoder function, but
with multiple calls to the
:meth:`~IncrementalEncoder.encode`/:meth:`~IncrementalDecoder.decode` method of
the incremental encoder/decoder. The incremental encoder/decoder keeps track of
the encoding/decoding process during method calls.

The joined output of calls to the
:meth:`~IncrementalEncoder.encode`/:meth:`~IncrementalDecoder.decode` method is
the same as if all the single inputs were joined into one, and this input was
encoded/decoded with the stateless encoder/decoder.


.. _incremental-encoder-objects:

IncrementalEncoder Objects
~~~~~~~~~~~~~~~~~~~~~~~~~~

The :class:`IncrementalEncoder` class is used for encoding an input in multiple
steps. It defines the following methods which every incremental encoder must
define in order to be compatible with the Python codec registry.


.. class:: IncrementalEncoder(errors='strict')

   Constructor for an :class:`IncrementalEncoder` instance.

   All incremental encoders must provide this constructor interface. They are free
   to add additional keyword arguments, but only the ones defined here are used by
   the Python codec registry.

   The :class:`IncrementalEncoder` may implement different error handling schemes
   by providing the *errors* keyword argument. See :ref:`error-handlers` for
   possible values.

   The *errors* argument will be assigned to an attribute of the same name.
   Assigning to this attribute makes it possible to switch between different error
   handling strategies during the lifetime of the :class:`IncrementalEncoder`
   object.


   .. method:: encode(object[, final])

      Encodes *object* (taking the current state of the encoder into account)
      and returns the resulting encoded object. If this is the last call to
      :meth:`encode` *final* must be true (the default is false).


   .. method:: reset()

      Reset the encoder to the initial state. The output is discarded: call
      ``.encode(object, final=True)``, passing an empty byte or text string
      if necessary, to reset the encoder and to get the output.


   .. method:: getstate()

      Return the current state of the encoder which must be an integer. The
      implementation should make sure that ``0`` is the most common
      state. (States that are more complicated than integers can be converted
      into an integer by marshaling/pickling the state and encoding the bytes
      of the resulting string into an integer).


   .. method:: setstate(state)

      Set the state of the encoder to *state*. *state* must be an encoder state
      returned by :meth:`getstate`.


.. _incremental-decoder-objects:

IncrementalDecoder Objects
~~~~~~~~~~~~~~~~~~~~~~~~~~

The :class:`IncrementalDecoder` class is used for decoding an input in multiple
steps. It defines the following methods which every incremental decoder must
define in order to be compatible with the Python codec registry.


.. class:: IncrementalDecoder(errors='strict')

   Constructor for an :class:`IncrementalDecoder` instance.

   All incremental decoders must provide this constructor interface. They are free
   to add additional keyword arguments, but only the ones defined here are used by
   the Python codec registry.

   The :class:`IncrementalDecoder` may implement different error handling schemes
   by providing the *errors* keyword argument. See :ref:`error-handlers` for
   possible values.

   The *errors* argument will be assigned to an attribute of the same name.
   Assigning to this attribute makes it possible to switch between different error
   handling strategies during the lifetime of the :class:`IncrementalDecoder`
   object.


   .. method:: decode(object[, final])

      Decodes *object* (taking the current state of the decoder into account)
      and returns the resulting decoded object. If this is the last call to
      :meth:`decode` *final* must be true (the default is false). If *final* is
      true the decoder must decode the input completely and must flush all
      buffers. If this isn't possible (e.g. because of incomplete byte sequences
      at the end of the input) it must initiate error handling just like in the
      stateless case (which might raise an exception).


   .. method:: reset()

      Reset the decoder to the initial state.


   .. method:: getstate()

      Return the current state of the decoder. This must be a tuple with two
      items, the first must be the buffer containing the still undecoded
      input. The second must be an integer and can be additional state
      info. (The implementation should make sure that ``0`` is the most common
      additional state info.) If this additional state info is ``0`` it must be
      possible to set the decoder to the state which has no input buffered and
      ``0`` as the additional state info, so that feeding the previously
      buffered input to the decoder returns it to the previous state without
      producing any output. (Additional state info that is more complicated than
      integers can be converted into an integer by marshaling/pickling the info
      and encoding the bytes of the resulting string into an integer.)


   .. method:: setstate(state)

      Set the state of the encoder to *state*. *state* must be a decoder state
      returned by :meth:`getstate`.


Stream Encoding and Decoding
^^^^^^^^^^^^^^^^^^^^^^^^^^^^


The :class:`StreamWriter` and :class:`StreamReader` classes provide generic
working interfaces which can be used to implement new encoding submodules very
easily. See :mod:`encodings.utf_8` for an example of how this is done.


.. _stream-writer-objects:

StreamWriter Objects
~~~~~~~~~~~~~~~~~~~~

The :class:`StreamWriter` class is a subclass of :class:`Codec` and defines the
following methods which every stream writer must define in order to be
compatible with the Python codec registry.


.. class:: StreamWriter(stream, errors='strict')

   Constructor for a :class:`StreamWriter` instance.

   All stream writers must provide this constructor interface. They are free to add
   additional keyword arguments, but only the ones defined here are used by the
   Python codec registry.

   The *stream* argument must be a file-like object open for writing
   text or binary data, as appropriate for the specific codec.

   The :class:`StreamWriter` may implement different error handling schemes by
   providing the *errors* keyword argument. See :ref:`error-handlers` for
   the standard error handlers the underlying stream codec may support.

   The *errors* argument will be assigned to an attribute of the same name.
   Assigning to this attribute makes it possible to switch between different error
   handling strategies during the lifetime of the :class:`StreamWriter` object.

   .. method:: write(object)

      Writes the object's contents encoded to the stream.


   .. method:: writelines(list)

      Writes the concatenated list of strings to the stream (possibly by reusing
      the :meth:`write` method). The standard bytes-to-bytes codecs
      do not support this method.


   .. method:: reset()

      Flushes and resets the codec buffers used for keeping state.

      Calling this method should ensure that the data on the output is put into
      a clean state that allows appending of new fresh data without having to
      rescan the whole stream to recover state.


In addition to the above methods, the :class:`StreamWriter` must also inherit
all other methods and attributes from the underlying stream.


.. _stream-reader-objects:

StreamReader Objects
~~~~~~~~~~~~~~~~~~~~

The :class:`StreamReader` class is a subclass of :class:`Codec` and defines the
following methods which every stream reader must define in order to be
compatible with the Python codec registry.


.. class:: StreamReader(stream, errors='strict')

   Constructor for a :class:`StreamReader` instance.

   All stream readers must provide this constructor interface. They are free to add
   additional keyword arguments, but only the ones defined here are used by the
   Python codec registry.

   The *stream* argument must be a file-like object open for reading
   text or binary data, as appropriate for the specific codec.

   The :class:`StreamReader` may implement different error handling schemes by
   providing the *errors* keyword argument. See :ref:`error-handlers` for
   the standard error handlers the underlying stream codec may support.

   The *errors* argument will be assigned to an attribute of the same name.
   Assigning to this attribute makes it possible to switch between different error
   handling strategies during the lifetime of the :class:`StreamReader` object.

   The set of allowed values for the *errors* argument can be extended with
   :func:`register_error`.


   .. method:: read([size[, chars, [firstline]]])

      Decodes data from the stream and returns the resulting object.

      The *chars* argument indicates the number of decoded
      code points or bytes to return. The :func:`read` method will
      never return more data than requested, but it might return less,
      if there is not enough available.

      The *size* argument indicates the approximate maximum
      number of encoded bytes or code points to read
      for decoding. The decoder can modify this setting as
      appropriate. The default value -1 indicates to read and decode as much as
      possible.  This parameter is intended to
      prevent having to decode huge files in one step.

      The *firstline* flag indicates that
      it would be sufficient to only return the first
      line, if there are decoding errors on later lines.

      The method should use a greedy read strategy meaning that it should read
      as much data as is allowed within the definition of the encoding and the
      given size, e.g.  if optional encoding endings or state markers are
      available on the stream, these should be read too.


   .. method:: readline([size[, keepends]])

      Read one line from the input stream and return the decoded data.

      *size*, if given, is passed as size argument to the stream's
      :meth:`read` method.

      If *keepends* is false line-endings will be stripped from the lines
      returned.


   .. method:: readlines([sizehint[, keepends]])

      Read all lines available on the input stream and return them as a list of
      lines.

      Line-endings are implemented using the codec's decoder method and are
      included in the list entries if *keepends* is true.

      *sizehint*, if given, is passed as the *size* argument to the stream's
      :meth:`read` method.


   .. method:: reset()

      Resets the codec buffers used for keeping state.

      Note that no stream repositioning should take place.  This method is
      primarily intended to be able to recover from decoding errors.


In addition to the above methods, the :class:`StreamReader` must also inherit
all other methods and attributes from the underlying stream.

.. _stream-reader-writer:

StreamReaderWriter Objects
~~~~~~~~~~~~~~~~~~~~~~~~~~

The :class:`StreamReaderWriter` is a convenience class that allows wrapping
streams which work in both read and write modes.

The design is such that one can use the factory functions returned by the
:func:`lookup` function to construct the instance.


.. class:: StreamReaderWriter(stream, Reader, Writer, errors='strict')

   Creates a :class:`StreamReaderWriter` instance. *stream* must be a file-like
   object. *Reader* and *Writer* must be factory functions or classes providing the
   :class:`StreamReader` and :class:`StreamWriter` interface resp. Error handling
   is done in the same way as defined for the stream readers and writers.

:class:`StreamReaderWriter` instances define the combined interfaces of
:class:`StreamReader` and :class:`StreamWriter` classes. They inherit all other
methods and attributes from the underlying stream.


.. _stream-recoder-objects:

StreamRecoder Objects
~~~~~~~~~~~~~~~~~~~~~

The :class:`StreamRecoder` translates data from one encoding to another,
which is sometimes useful when dealing with different encoding environments.

The design is such that one can use the factory functions returned by the
:func:`lookup` function to construct the instance.


.. class:: StreamRecoder(stream, encode, decode, Reader, Writer, errors='strict')

   Creates a :class:`StreamRecoder` instance which implements a two-way conversion:
   *encode* and *decode* work on the frontend — the data visible to
   code calling :meth:`read` and :meth:`write`, while *Reader* and *Writer*
   work on the backend — the data in *stream*.

   You can use these objects to do transparent transcodings from e.g. Latin-1
   to UTF-8 and back.

   The *stream* argument must be a file-like object.

   The *encode* and *decode* arguments must
   adhere to the :class:`Codec` interface. *Reader* and
   *Writer* must be factory functions or classes providing objects of the
   :class:`StreamReader` and :class:`StreamWriter` interface respectively.

   Error handling is done in the same way as defined for the stream readers and
   writers.


:class:`StreamRecoder` instances define the combined interfaces of
:class:`StreamReader` and :class:`StreamWriter` classes. They inherit all other
methods and attributes from the underlying stream.


.. _encodings-overview:

Encodings and Unicode
---------------------

Strings are stored internally as sequences of code points in
range ``0x0``--``0x10FFFF``.  (See :pep:`393` for
more details about the implementation.)
Once a string object is used outside of CPU and memory, endianness
and how these arrays are stored as bytes become an issue.  As with other
codecs, serialising a string into a sequence of bytes is known as *encoding*,
and recreating the string from the sequence of bytes is known as *decoding*.
There are a variety of different text serialisation codecs, which are
collectivity referred to as :term:`text encodings <text encoding>`.

The simplest text encoding (called ``'latin-1'`` or ``'iso-8859-1'``) maps
the code points 0--255 to the bytes ``0x0``--``0xff``, which means that a string
object that contains code points above ``U+00FF`` can't be encoded with this
codec. Doing so will raise a :exc:`UnicodeEncodeError` that looks
like the following (although the details of the error message may differ):
``UnicodeEncodeError: 'latin-1' codec can't encode character '\u1234' in
position 3: ordinal not in range(256)``.

There's another group of encodings (the so called charmap encodings) that choose
a different subset of all Unicode code points and how these code points are
mapped to the bytes ``0x0``--``0xff``. To see how this is done simply open
e.g. :file:`encodings/cp1252.py` (which is an encoding that is used primarily on
Windows). There's a string constant with 256 characters that shows you which
character is mapped to which byte value.

All of these encodings can only encode 256 of the 1114112 code points
defined in Unicode. A simple and straightforward way that can store each Unicode
code point, is to store each code point as four consecutive bytes. There are two
possibilities: store the bytes in big endian or in little endian order. These
two encodings are called ``UTF-32-BE`` and ``UTF-32-LE`` respectively. Their
disadvantage is that if e.g. you use ``UTF-32-BE`` on a little endian machine you
will always have to swap bytes on encoding and decoding. ``UTF-32`` avoids this
problem: bytes will always be in natural endianness. When these bytes are read
by a CPU with a different endianness, then bytes have to be swapped though. To
be able to detect the endianness of a ``UTF-16`` or ``UTF-32`` byte sequence,
there's the so called BOM ("Byte Order Mark"). This is the Unicode character
``U+FEFF``. This character can be prepended to every ``UTF-16`` or ``UTF-32``
byte sequence. The byte swapped version of this character (``0xFFFE``) is an
illegal character that may not appear in a Unicode text. So when the
first character in an ``UTF-16`` or ``UTF-32`` byte sequence
appears to be a ``U+FFFE`` the bytes have to be swapped on decoding.
Unfortunately the character ``U+FEFF`` had a second purpose as
a ``ZERO WIDTH NO-BREAK SPACE``: a character that has no width and doesn't allow
a word to be split. It can e.g. be used to give hints to a ligature algorithm.
With Unicode 4.0 using ``U+FEFF`` as a ``ZERO WIDTH NO-BREAK SPACE`` has been
deprecated (with ``U+2060`` (``WORD JOINER``) assuming this role). Nevertheless
Unicode software still must be able to handle ``U+FEFF`` in both roles: as a BOM
it's a device to determine the storage layout of the encoded bytes, and vanishes
once the byte sequence has been decoded into a string; as a ``ZERO WIDTH
NO-BREAK SPACE`` it's a normal character that will be decoded like any other.

There's another encoding that is able to encoding the full range of Unicode
characters: UTF-8. UTF-8 is an 8-bit encoding, which means there are no issues
with byte order in UTF-8. Each byte in a UTF-8 byte sequence consists of two
parts: marker bits (the most significant bits) and payload bits. The marker bits
are a sequence of zero to four ``1`` bits followed by a ``0`` bit. Unicode characters are
encoded like this (with x being payload bits, which when concatenated give the
Unicode character):

+-----------------------------------+----------------------------------------------+
| Range                             | Encoding                                     |
+===================================+==============================================+
| ``U-00000000`` ... ``U-0000007F`` | 0xxxxxxx                                     |
+-----------------------------------+----------------------------------------------+
| ``U-00000080`` ... ``U-000007FF`` | 110xxxxx 10xxxxxx                            |
+-----------------------------------+----------------------------------------------+
| ``U-00000800`` ... ``U-0000FFFF`` | 1110xxxx 10xxxxxx 10xxxxxx                   |
+-----------------------------------+----------------------------------------------+
| ``U-00010000`` ... ``U-0010FFFF`` | 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx          |
+-----------------------------------+----------------------------------------------+

The least significant bit of the Unicode character is the rightmost x bit.

As UTF-8 is an 8-bit encoding no BOM is required and any ``U+FEFF`` character in
the decoded string (even if it's the first character) is treated as a ``ZERO
WIDTH NO-BREAK SPACE``.

Without external information it's impossible to reliably determine which
encoding was used for encoding a string. Each charmap encoding can
decode any random byte sequence. However that's not possible with UTF-8, as
UTF-8 byte sequences have a structure that doesn't allow arbitrary byte
sequences. To increase the reliability with which a UTF-8 encoding can be
detected, Microsoft invented a variant of UTF-8 (that Python 2.5 calls
``"utf-8-sig"``) for its Notepad program: Before any of the Unicode characters
is written to the file, a UTF-8 encoded BOM (which looks like this as a byte
sequence: ``0xef``, ``0xbb``, ``0xbf``) is written. As it's rather improbable
that any charmap encoded file starts with these byte values (which would e.g.
map to

   | LATIN SMALL LETTER I WITH DIAERESIS
   | RIGHT-POINTING DOUBLE ANGLE QUOTATION MARK
   | INVERTED QUESTION MARK

in iso-8859-1), this increases the probability that a ``utf-8-sig`` encoding can be
correctly guessed from the byte sequence. So here the BOM is not used to be able
to determine the byte order used for generating the byte sequence, but as a
signature that helps in guessing the encoding. On encoding the utf-8-sig codec
will write ``0xef``, ``0xbb``, ``0xbf`` as the first three bytes to the file. On
decoding ``utf-8-sig`` will skip those three bytes if they appear as the first
three bytes in the file.  In UTF-8, the use of the BOM is discouraged and
should generally be avoided.


.. _standard-encodings:

Standard Encodings
------------------

Python comes with a number of codecs built-in, either implemented as C functions
or with dictionaries as mapping tables. The following table lists the codecs by
name, together with a few common aliases, and the languages for which the
encoding is likely used. Neither the list of aliases nor the list of languages
is meant to be exhaustive. Notice that spelling alternatives that only differ in
case or use a hyphen instead of an underscore are also valid aliases; therefore,
e.g. ``'utf-8'`` is a valid alias for the ``'utf_8'`` codec.

.. impl-detail::

   Some common encodings can bypass the codecs lookup machinery to
   improve performance.  These optimization opportunities are only
   recognized by CPython for a limited set of (case insensitive)
   aliases: utf-8, utf8, latin-1, latin1, iso-8859-1, iso8859-1, mbcs
   (Windows only), ascii, us-ascii, utf-16, utf16, utf-32, utf32, and
   the same using underscores instead of dashes. Using alternative
   aliases for these encodings may result in slower execution.

   .. versionchanged:: 3.6
      Optimization opportunity recognized for us-ascii.

Many of the character sets support the same languages. They vary in individual
characters (e.g. whether the EURO SIGN is supported or not), and in the
assignment of characters to code positions. For the European languages in
particular, the following variants typically exist:

* an ISO 8859 codeset

* a Microsoft Windows code page, which is typically derived from an 8859 codeset,
  but replaces control characters with additional graphic characters

* an IBM EBCDIC code page

* an IBM PC code page, which is ASCII compatible

.. tabularcolumns:: |l|p{0.3\linewidth}|p{0.3\linewidth}|

+-----------------+--------------------------------+--------------------------------+
| Codec           | Aliases                        | Languages                      |
+=================+================================+================================+
| ascii           | 646, us-ascii                  | English                        |
+-----------------+--------------------------------+--------------------------------+
| big5            | big5-tw, csbig5                | Traditional Chinese            |
+-----------------+--------------------------------+--------------------------------+
| big5hkscs       | big5-hkscs, hkscs              | Traditional Chinese            |
+-----------------+--------------------------------+--------------------------------+
| cp037           | IBM037, IBM039                 | English                        |
+-----------------+--------------------------------+--------------------------------+
| cp273           | 273, IBM273, csIBM273          | German                         |
|                 |                                |                                |
|                 |                                | .. versionadded:: 3.4          |
+-----------------+--------------------------------+--------------------------------+
| cp424           | EBCDIC-CP-HE, IBM424           | Hebrew                         |
+-----------------+--------------------------------+--------------------------------+
| cp437           | 437, IBM437                    | English                        |
+-----------------+--------------------------------+--------------------------------+
| cp500           | EBCDIC-CP-BE, EBCDIC-CP-CH,    | Western Europe                 |
|                 | IBM500                         |                                |
+-----------------+--------------------------------+--------------------------------+
| cp720           |                                | Arabic                         |
+-----------------+--------------------------------+--------------------------------+
| cp737           |                                | Greek                          |
+-----------------+--------------------------------+--------------------------------+
| cp775           | IBM775                         | Baltic languages               |
+-----------------+--------------------------------+--------------------------------+
| cp850           | 850, IBM850                    | Western Europe                 |
+-----------------+--------------------------------+--------------------------------+
| cp852           | 852, IBM852                    | Central and Eastern Europe     |
+-----------------+--------------------------------+--------------------------------+
| cp855           | 855, IBM855                    | Bulgarian, Byelorussian,       |
|                 |                                | Macedonian, Russian, Serbian   |
+-----------------+--------------------------------+--------------------------------+
| cp856           |                                | Hebrew                         |
+-----------------+--------------------------------+--------------------------------+
| cp857           | 857, IBM857                    | Turkish                        |
+-----------------+--------------------------------+--------------------------------+
| cp858           | 858, IBM858                    | Western Europe                 |
+-----------------+--------------------------------+--------------------------------+
| cp860           | 860, IBM860                    | Portuguese                     |
+-----------------+--------------------------------+--------------------------------+
| cp861           | 861, CP-IS, IBM861             | Icelandic                      |
+-----------------+--------------------------------+--------------------------------+
| cp862           | 862, IBM862                    | Hebrew                         |
+-----------------+--------------------------------+--------------------------------+
| cp863           | 863, IBM863                    | Canadian                       |
+-----------------+--------------------------------+--------------------------------+
| cp864           | IBM864                         | Arabic                         |
+-----------------+--------------------------------+--------------------------------+
| cp865           | 865, IBM865                    | Danish, Norwegian              |
+-----------------+--------------------------------+--------------------------------+
| cp866           | 866, IBM866                    | Russian                        |
+-----------------+--------------------------------+--------------------------------+
| cp869           | 869, CP-GR, IBM869             | Greek                          |
+-----------------+--------------------------------+--------------------------------+
| cp874           |                                | Thai                           |
+-----------------+--------------------------------+--------------------------------+
| cp875           |                                | Greek                          |
+-----------------+--------------------------------+--------------------------------+
| cp932           | 932, ms932, mskanji, ms-kanji  | Japanese                       |
+-----------------+--------------------------------+--------------------------------+
| cp949           | 949, ms949, uhc                | Korean                         |
+-----------------+--------------------------------+--------------------------------+
| cp950           | 950, ms950                     | Traditional Chinese            |
+-----------------+--------------------------------+--------------------------------+
| cp1006          |                                | Urdu                           |
+-----------------+--------------------------------+--------------------------------+
| cp1026          | ibm1026                        | Turkish                        |
+-----------------+--------------------------------+--------------------------------+
| cp1125          | 1125, ibm1125, cp866u, ruscii  | Ukrainian                      |
|                 |                                |                                |
|                 |                                | .. versionadded:: 3.4          |
+-----------------+--------------------------------+--------------------------------+
| cp1140          | ibm1140                        | Western Europe                 |
+-----------------+--------------------------------+--------------------------------+
| cp1250          | windows-1250                   | Central and Eastern Europe     |
+-----------------+--------------------------------+--------------------------------+
| cp1251          | windows-1251                   | Bulgarian, Byelorussian,       |
|                 |                                | Macedonian, Russian, Serbian   |
+-----------------+--------------------------------+--------------------------------+
| cp1252          | windows-1252                   | Western Europe                 |
+-----------------+--------------------------------+--------------------------------+
| cp1253          | windows-1253                   | Greek                          |
+-----------------+--------------------------------+--------------------------------+
| cp1254          | windows-1254                   | Turkish                        |
+-----------------+--------------------------------+--------------------------------+
| cp1255          | windows-1255                   | Hebrew                         |
+-----------------+--------------------------------+--------------------------------+
| cp1256          | windows-1256                   | Arabic                         |
+-----------------+--------------------------------+--------------------------------+
| cp1257          | windows-1257                   | Baltic languages               |
+-----------------+--------------------------------+--------------------------------+
| cp1258          | windows-1258                   | Vietnamese                     |
+-----------------+--------------------------------+--------------------------------+
| cp65001         |                                | Windows only: Windows UTF-8    |
|                 |                                | (``CP_UTF8``)                  |
|                 |                                |                                |
|                 |                                | .. versionadded:: 3.3          |
+-----------------+--------------------------------+--------------------------------+
| euc_jp          | eucjp, ujis, u-jis             | Japanese                       |
+-----------------+--------------------------------+--------------------------------+
| euc_jis_2004    | jisx0213, eucjis2004           | Japanese                       |
+-----------------+--------------------------------+--------------------------------+
| euc_jisx0213    | eucjisx0213                    | Japanese                       |
+-----------------+--------------------------------+--------------------------------+
| euc_kr          | euckr, korean, ksc5601,        | Korean                         |
|                 | ks_c-5601, ks_c-5601-1987,     |                                |
|                 | ksx1001, ks_x-1001             |                                |
+-----------------+--------------------------------+--------------------------------+
| gb2312          | chinese, csiso58gb231280, euc- | Simplified Chinese             |
|                 | cn, euccn, eucgb2312-cn,       |                                |
|                 | gb2312-1980, gb2312-80, iso-   |                                |
|                 | ir-58                          |                                |
+-----------------+--------------------------------+--------------------------------+
| gbk             | 936, cp936, ms936              | Unified Chinese                |
+-----------------+--------------------------------+--------------------------------+
| gb18030         | gb18030-2000                   | Unified Chinese                |
+-----------------+--------------------------------+--------------------------------+
| hz              | hzgb, hz-gb, hz-gb-2312        | Simplified Chinese             |
+-----------------+--------------------------------+--------------------------------+
| iso2022_jp      | csiso2022jp, iso2022jp,        | Japanese                       |
|                 | iso-2022-jp                    |                                |
+-----------------+--------------------------------+--------------------------------+
| iso2022_jp_1    | iso2022jp-1, iso-2022-jp-1     | Japanese                       |
+-----------------+--------------------------------+--------------------------------+
| iso2022_jp_2    | iso2022jp-2, iso-2022-jp-2     | Japanese, Korean, Simplified   |
|                 |                                | Chinese, Western Europe, Greek |
+-----------------+--------------------------------+--------------------------------+
| iso2022_jp_2004 | iso2022jp-2004,                | Japanese                       |
|                 | iso-2022-jp-2004               |                                |
+-----------------+--------------------------------+--------------------------------+
| iso2022_jp_3    | iso2022jp-3, iso-2022-jp-3     | Japanese                       |
+-----------------+--------------------------------+--------------------------------+
| iso2022_jp_ext  | iso2022jp-ext, iso-2022-jp-ext | Japanese                       |
+-----------------+--------------------------------+--------------------------------+
| iso2022_kr      | csiso2022kr, iso2022kr,        | Korean                         |
|                 | iso-2022-kr                    |                                |
+-----------------+--------------------------------+--------------------------------+
| latin_1         | iso-8859-1, iso8859-1, 8859,   | West Europe                    |
|                 | cp819, latin, latin1, L1       |                                |
+-----------------+--------------------------------+--------------------------------+
| iso8859_2       | iso-8859-2, latin2, L2         | Central and Eastern Europe     |
+-----------------+--------------------------------+--------------------------------+
| iso8859_3       | iso-8859-3, latin3, L3         | Esperanto, Maltese             |
+-----------------+--------------------------------+--------------------------------+
| iso8859_4       | iso-8859-4, latin4, L4         | Baltic languages               |
+-----------------+--------------------------------+--------------------------------+
| iso8859_5       | iso-8859-5, cyrillic           | Bulgarian, Byelorussian,       |
|                 |                                | Macedonian, Russian, Serbian   |
+-----------------+--------------------------------+--------------------------------+
| iso8859_6       | iso-8859-6, arabic             | Arabic                         |
+-----------------+--------------------------------+--------------------------------+
| iso8859_7       | iso-8859-7, greek, greek8      | Greek                          |
+-----------------+--------------------------------+--------------------------------+
| iso8859_8       | iso-8859-8, hebrew             | Hebrew                         |
+-----------------+--------------------------------+--------------------------------+
| iso8859_9       | iso-8859-9, latin5, L5         | Turkish                        |
+-----------------+--------------------------------+--------------------------------+
| iso8859_10      | iso-8859-10, latin6, L6        | Nordic languages               |
+-----------------+--------------------------------+--------------------------------+
| iso8859_11      | iso-8859-11, thai              | Thai languages                 |
+-----------------+--------------------------------+--------------------------------+
| iso8859_13      | iso-8859-13, latin7, L7        | Baltic languages               |
+-----------------+--------------------------------+--------------------------------+
| iso8859_14      | iso-8859-14, latin8, L8        | Celtic languages               |
+-----------------+--------------------------------+--------------------------------+
| iso8859_15      | iso-8859-15, latin9, L9        | Western Europe                 |
+-----------------+--------------------------------+--------------------------------+
| iso8859_16      | iso-8859-16, latin10, L10      | South-Eastern Europe           |
+-----------------+--------------------------------+--------------------------------+
| johab           | cp1361, ms1361                 | Korean                         |
+-----------------+--------------------------------+--------------------------------+
| koi8_r          |                                | Russian                        |
+-----------------+--------------------------------+--------------------------------+
| koi8_t          |                                | Tajik                          |
|                 |                                |                                |
|                 |                                | .. versionadded:: 3.5          |
+-----------------+--------------------------------+--------------------------------+
| koi8_u          |                                | Ukrainian                      |
+-----------------+--------------------------------+--------------------------------+
| kz1048          | kz_1048, strk1048_2002, rk1048 | Kazakh                         |
|                 |                                |                                |
|                 |                                | .. versionadded:: 3.5          |
+-----------------+--------------------------------+--------------------------------+
| mac_cyrillic    | maccyrillic                    | Bulgarian, Byelorussian,       |
|                 |                                | Macedonian, Russian, Serbian   |
+-----------------+--------------------------------+--------------------------------+
| mac_greek       | macgreek                       | Greek                          |
+-----------------+--------------------------------+--------------------------------+
| mac_iceland     | maciceland                     | Icelandic                      |
+-----------------+--------------------------------+--------------------------------+
| mac_latin2      | maclatin2, maccentraleurope    | Central and Eastern Europe     |
+-----------------+--------------------------------+--------------------------------+
| mac_roman       | macroman, macintosh            | Western Europe                 |
+-----------------+--------------------------------+--------------------------------+
| mac_turkish     | macturkish                     | Turkish                        |
+-----------------+--------------------------------+--------------------------------+
| ptcp154         | csptcp154, pt154, cp154,       | Kazakh                         |
|                 | cyrillic-asian                 |                                |
+-----------------+--------------------------------+--------------------------------+
| shift_jis       | csshiftjis, shiftjis, sjis,    | Japanese                       |
|                 | s_jis                          |                                |
+-----------------+--------------------------------+--------------------------------+
| shift_jis_2004  | shiftjis2004, sjis_2004,       | Japanese                       |
|                 | sjis2004                       |                                |
+-----------------+--------------------------------+--------------------------------+
| shift_jisx0213  | shiftjisx0213, sjisx0213,      | Japanese                       |
|                 | s_jisx0213                     |                                |
+-----------------+--------------------------------+--------------------------------+
| utf_32          | U32, utf32                     | all languages                  |
+-----------------+--------------------------------+--------------------------------+
| utf_32_be       | UTF-32BE                       | all languages                  |
+-----------------+--------------------------------+--------------------------------+
| utf_32_le       | UTF-32LE                       | all languages                  |
+-----------------+--------------------------------+--------------------------------+
| utf_16          | U16, utf16                     | all languages                  |
+-----------------+--------------------------------+--------------------------------+
| utf_16_be       | UTF-16BE                       | all languages                  |
+-----------------+--------------------------------+--------------------------------+
| utf_16_le       | UTF-16LE                       | all languages                  |
+-----------------+--------------------------------+--------------------------------+
| utf_7           | U7, unicode-1-1-utf-7          | all languages                  |
+-----------------+--------------------------------+--------------------------------+
| utf_8           | U8, UTF, utf8                  | all languages                  |
+-----------------+--------------------------------+--------------------------------+
| utf_8_sig       |                                | all languages                  |
+-----------------+--------------------------------+--------------------------------+

.. versionchanged:: 3.4
   The utf-16\* and utf-32\* encoders no longer allow surrogate code points
   (``U+D800``--``U+DFFF``) to be encoded.
   The utf-32\* decoders no longer decode
   byte sequences that correspond to surrogate code points.


Python Specific Encodings
-------------------------

A number of predefined codecs are specific to Python, so their codec names have
no meaning outside Python.  These are listed in the tables below based on the
expected input and output types (note that while text encodings are the most
common use case for codecs, the underlying codec infrastructure supports
arbitrary data transforms rather than just text encodings).  For asymmetric
codecs, the stated purpose describes the encoding direction.

Text Encodings
^^^^^^^^^^^^^^

The following codecs provide :class:`str` to :class:`bytes` encoding and
:term:`bytes-like object` to :class:`str` decoding, similar to the Unicode text
encodings.

.. tabularcolumns:: |l|p{0.3\linewidth}|p{0.3\linewidth}|

+--------------------+---------+---------------------------+
| Codec              | Aliases | Purpose                   |
+====================+=========+===========================+
| idna               |         | Implements :rfc:`3490`,   |
|                    |         | see also                  |
|                    |         | :mod:`encodings.idna`.    |
|                    |         | Only ``errors='strict'``  |
|                    |         | is supported.             |
+--------------------+---------+---------------------------+
| mbcs               | ansi,   | Windows only: Encode      |
|                    | dbcs    | operand according to the  |
|                    |         | ANSI codepage (CP_ACP)    |
+--------------------+---------+---------------------------+
| oem                |         | Windows only: Encode      |
|                    |         | operand according to the  |
|                    |         | OEM codepage (CP_OEMCP)   |
|                    |         |                           |
|                    |         | .. versionadded:: 3.6     |
+--------------------+---------+---------------------------+
| palmos             |         | Encoding of PalmOS 3.5    |
+--------------------+---------+---------------------------+
| punycode           |         | Implements :rfc:`3492`.   |
|                    |         | Stateful codecs are not   |
|                    |         | supported.                |
+--------------------+---------+---------------------------+
| raw_unicode_escape |         | Latin-1 encoding with     |
|                    |         | ``\uXXXX`` and            |
|                    |         | ``\UXXXXXXXX`` for other  |
|                    |         | code points. Existing     |
|                    |         | backslashes are not       |
|                    |         | escaped in any way.       |
|                    |         | It is used in the Python  |
|                    |         | pickle protocol.          |
+--------------------+---------+---------------------------+
| undefined          |         | Raise an exception for    |
|                    |         | all conversions, even     |
|                    |         | empty strings. The error  |
|                    |         | handler is ignored.       |
+--------------------+---------+---------------------------+
| unicode_escape     |         | Encoding suitable as the  |
|                    |         | contents of a Unicode     |
|                    |         | literal in ASCII-encoded  |
|                    |         | Python source code,       |
|                    |         | except that quotes are    |
|                    |         | not escaped. Decodes from |
|                    |         | Latin-1 source code.      |
|                    |         | Beware that Python source |
|                    |         | code actually uses UTF-8  |
|                    |         | by default.               |
+--------------------+---------+---------------------------+
| unicode_internal   |         | Return the internal       |
|                    |         | representation of the     |
|                    |         | operand. Stateful codecs  |
|                    |         | are not supported.        |
|                    |         |                           |
|                    |         | .. deprecated:: 3.3       |
|                    |         |    This representation is |
|                    |         |    obsoleted by           |
|                    |         |    :pep:`393`.            |
+--------------------+---------+---------------------------+

.. _binary-transforms:

Binary Transforms
^^^^^^^^^^^^^^^^^

The following codecs provide binary transforms: :term:`bytes-like object`
to :class:`bytes` mappings.  They are not supported by :meth:`bytes.decode`
(which only produces :class:`str` output).


.. tabularcolumns:: |l|L|L|L|

+----------------------+------------------+------------------------------+------------------------------+
| Codec                | Aliases          | Purpose                      | Encoder / decoder            |
+======================+==================+==============================+==============================+
| base64_codec [#b64]_ | base64, base_64  | Convert operand to multiline | :meth:`base64.encodebytes` / |
|                      |                  | MIME base64 (the result      | :meth:`base64.decodebytes`   |
|                      |                  | always includes a trailing   |                              |
|                      |                  | ``'\n'``)                    |                              |
|                      |                  |                              |                              |
|                      |                  | .. versionchanged:: 3.4      |                              |
|                      |                  |    accepts any               |                              |
|                      |                  |    :term:`bytes-like object` |                              |
|                      |                  |    as input for encoding and |                              |
|                      |                  |    decoding                  |                              |
+----------------------+------------------+------------------------------+------------------------------+
| bz2_codec            | bz2              | Compress the operand         | :meth:`bz2.compress` /       |
|                      |                  | using bz2                    | :meth:`bz2.decompress`       |
+----------------------+------------------+------------------------------+------------------------------+
| hex_codec            | hex              | Convert operand to           | :meth:`binascii.b2a_hex` /   |
|                      |                  | hexadecimal                  | :meth:`binascii.a2b_hex`     |
|                      |                  | representation, with two     |                              |
|                      |                  | digits per byte              |                              |
+----------------------+------------------+------------------------------+------------------------------+
| quopri_codec         | quopri,          | Convert operand to MIME      | :meth:`quopri.encode` with   |
|                      | quotedprintable, | quoted printable             | ``quotetabs=True`` /         |
|                      | quoted_printable |                              | :meth:`quopri.decode`        |
+----------------------+------------------+------------------------------+------------------------------+
| uu_codec             | uu               | Convert the operand using    | :meth:`uu.encode` /          |
|                      |                  | uuencode                     | :meth:`uu.decode`            |
+----------------------+------------------+------------------------------+------------------------------+
| zlib_codec           | zip, zlib        | Compress the operand         | :meth:`zlib.compress` /      |
|                      |                  | using gzip                   | :meth:`zlib.decompress`      |
+----------------------+------------------+------------------------------+------------------------------+

.. [#b64] In addition to :term:`bytes-like objects <bytes-like object>`,
   ``'base64_codec'`` also accepts ASCII-only instances of :class:`str` for
   decoding

.. versionadded:: 3.2
   Restoration of the binary transforms.

.. versionchanged:: 3.4
   Restoration of the aliases for the binary transforms.


.. _text-transforms:

Text Transforms
^^^^^^^^^^^^^^^

The following codec provides a text transform: a :class:`str` to :class:`str`
mapping.  It is not supported by :meth:`str.encode` (which only produces
:class:`bytes` output).

.. tabularcolumns:: |l|l|L|

+--------------------+---------+---------------------------+
| Codec              | Aliases | Purpose                   |
+====================+=========+===========================+
| rot_13             | rot13   | Returns the Caesar-cypher |
|                    |         | encryption of the operand |
+--------------------+---------+---------------------------+

.. versionadded:: 3.2
   Restoration of the ``rot_13`` text transform.

.. versionchanged:: 3.4
   Restoration of the ``rot13`` alias.


:mod:`encodings.idna` --- Internationalized Domain Names in Applications
------------------------------------------------------------------------

.. module:: encodings.idna
   :synopsis: Internationalized Domain Names implementation
.. moduleauthor:: Martin v. Löwis

This module implements :rfc:`3490` (Internationalized Domain Names in
Applications) and :rfc:`3492` (Nameprep: A Stringprep Profile for
Internationalized Domain Names (IDN)). It builds upon the ``punycode`` encoding
and :mod:`stringprep`.

These RFCs together define a protocol to support non-ASCII characters in domain
names. A domain name containing non-ASCII characters (such as
``www.Alliancefrançaise.nu``) is converted into an ASCII-compatible encoding
(ACE, such as ``www.xn--alliancefranaise-npb.nu``). The ACE form of the domain
name is then used in all places where arbitrary characters are not allowed by
the protocol, such as DNS queries, HTTP :mailheader:`Host` fields, and so
on. This conversion is carried out in the application; if possible invisible to
the user: The application should transparently convert Unicode domain labels to
IDNA on the wire, and convert back ACE labels to Unicode before presenting them
to the user.

Python supports this conversion in several ways:  the ``idna`` codec performs
conversion between Unicode and ACE, separating an input string into labels
based on the separator characters defined in :rfc:`section 3.1 of RFC 3490 <3490#section-3.1>`
and converting each label to ACE as required, and conversely separating an input
byte string into labels based on the ``.`` separator and converting any ACE
labels found into unicode.  Furthermore, the :mod:`socket` module
transparently converts Unicode host names to ACE, so that applications need not
be concerned about converting host names themselves when they pass them to the
socket module. On top of that, modules that have host names as function
parameters, such as :mod:`http.client` and :mod:`ftplib`, accept Unicode host
names (:mod:`http.client` then also transparently sends an IDNA hostname in the
:mailheader:`Host` field if it sends that field at all).

When receiving host names from the wire (such as in reverse name lookup), no
automatic conversion to Unicode is performed: Applications wishing to present
such host names to the user should decode them to Unicode.

The module :mod:`encodings.idna` also implements the nameprep procedure, which
performs certain normalizations on host names, to achieve case-insensitivity of
international domain names, and to unify similar characters. The nameprep
functions can be used directly if desired.


.. function:: nameprep(label)

   Return the nameprepped version of *label*. The implementation currently assumes
   query strings, so ``AllowUnassigned`` is true.


.. function:: ToASCII(label)

   Convert a label to ASCII, as specified in :rfc:`3490`. ``UseSTD3ASCIIRules`` is
   assumed to be false.


.. function:: ToUnicode(label)

   Convert a label to Unicode, as specified in :rfc:`3490`.


:mod:`encodings.mbcs` --- Windows ANSI codepage
-----------------------------------------------

.. module:: encodings.mbcs
   :synopsis: Windows ANSI codepage

Encode operand according to the ANSI codepage (CP_ACP).

.. availability:: Windows only.

.. versionchanged:: 3.3
   Support any error handler.

.. versionchanged:: 3.2
   Before 3.2, the *errors* argument was ignored; ``'replace'`` was always used
   to encode, and ``'ignore'`` to decode.


:mod:`encodings.utf_8_sig` --- UTF-8 codec with BOM signature
-------------------------------------------------------------

.. module:: encodings.utf_8_sig
   :synopsis: UTF-8 codec with BOM signature
.. moduleauthor:: Walter Dörwald

This module implements a variant of the UTF-8 codec: On encoding a UTF-8 encoded
BOM will be prepended to the UTF-8 encoded bytes. For the stateful encoder this
is only done once (on the first write to the byte stream).  For decoding an
optional UTF-8 encoded BOM at the start of the data will be skipped.