: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 -1 which means that the default buffer size will be used.


.. 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 decoder 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()

      Resets the codec buffers used for keeping internal 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 :meth:`decode` 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 internal 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, e.g., from 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 vers                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    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