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:mod:`!codecs` --- Codec registry and base classes
==================================================
.. module:: codecs
:synopsis: Encode and decode data and streams.
**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 (and
decode bytes to text), 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 be used 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.
.. function:: charmap_build(string)
Return a mapping suitable for encoding with a custom single-byte encoding.
Given a :class:`str` *string* of up to 256 characters representing a
decoding table, returns either a compact internal mapping object
``EncodingMap`` or a :class:`dictionary <dict>` mapping character ordinals
to byte values. Raises a :exc:`TypeError` on invalid input.
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.
This function first normalizes the *encoding*: all ASCII letters are
converted to lower case, spaces are replaced with hyphens.
Then encoding is 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.
.. versionchanged:: 3.9
Any characters except ASCII letters and digits and a dot are converted to underscore.
.. versionchanged:: 3.15
No characters are converted to underscore anymore.
Spaces are converted to hyphens.
.. 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 with spaces
converted to hyphens, and return a :class:`CodecInfo` object.
In case a search function cannot find a given encoding, it should return
``None``.
.. function:: unregister(search_function, /)
Unregister a codec search function and clear the registry's cache.
If the search function is not registered, do nothing.
.. versionadded:: 3.10
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::
If *encoding* is not ``None``, then the
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.
.. versionchanged:: 3.11
The ``'U'`` mode has been removed.
.. deprecated:: 3.14
:func:`codecs.open` has been superseded by :func:`open`.
.. 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*. *iterator* must yield :class:`str` objects.
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*. *iterator* must yield :class:`bytes` objects.
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`.
.. function:: readbuffer_encode(buffer, errors=None, /)
Return a :class:`tuple` containing the raw bytes of *buffer*, a
:ref:`buffer-compatible object <bufferobjects>` or :class:`str`
(encoded to UTF-8 before processing), and their length in bytes.
The *errors* argument is ignored.
.. code-block:: pycon
>>> codecs.readbuffer_encode(b"Zito")
(b'Zito', 4)
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:
>>> 'German ß, ♬'.encode(encoding='ascii', errors='backslashreplace')
b'German \\xdf, \\u266c'
>>> 'German ß, ♬'.encode(encoding='ascii', errors='xmlcharrefreplace')
b'German ß, ♬'
.. index::
pair: strict; error handler's name
pair: ignore; error handler's name
pair: replace; error handler's name
pair: backslashreplace; error handler's name
pair: surrogateescape; error handler's name
single: ? (question mark); replacement character
single: \ (backslash); escape sequence
single: \x; escape sequence
single: \u; escape sequence
single: \U; escape sequence
The following error handlers can be used with all Python
:ref:`standard-encodings` 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`. |
+-------------------------+-----------------------------------------------+
| ``'replace'`` | Replace with a replacement marker. On |
| | encoding, use ``?`` (ASCII character). On |
| | decoding, use ``�`` (U+FFFD, the official |
| | REPLACEMENT CHARACTER). Implemented in |
| | :func:`replace_errors`. |
+-------------------------+-----------------------------------------------+
| ``'backslashreplace'`` | Replace with backslashed escape sequences. |
| | On encoding, use hexadecimal form of Unicode |
| | code point with formats :samp:`\\x{hh}` |
| | :samp:`\\u{xxxx}` :samp:`\\U{xxxxxxxx}`. |
| | On decoding, use hexadecimal form of byte |
| | value with format :samp:`\\x{hh}`. |
| | Implemented in |
| | :func:`backslashreplace_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.) |
+-------------------------+-----------------------------------------------+
.. index::
pair: xmlcharrefreplace; error handler's name
pair: namereplace; error handler's name
single: \N; escape sequence
The following error handlers are only applicable to encoding (within
:term:`text encodings <text encoding>`):
+-------------------------+-----------------------------------------------+
| Value | Meaning |
+=========================+===============================================+
| ``'xmlcharrefreplace'`` | Replace with XML/HTML numeric character |
| | reference, which is a decimal form of Unicode |
| | code point with format :samp:`&#{num};`. |
| | Implemented in |
| | :func:`xmlcharrefreplace_errors`. |
+-------------------------+-----------------------------------------------+
| ``'namereplace'`` | Replace with ``\N{...}`` escape sequences, |
| | what appears in the braces is the Name |
| | property from Unicode Character Database. |
| | Implemented in :func:`namereplace_errors`. |
+-------------------------+-----------------------------------------------+
.. index::
pair: surrogatepass; error handler's name
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 surrogate code|
| | utf-16-be, utf-16-le, | point (``U+D800`` - ``U+DFFF``) as normal |
| | utf-32-be, utf-32-le | code point. Otherwise these codecs treat |
| | | the presence of surrogate code point in |
| | | :class:`str` as an error. |
+-------------------+------------------------+-------------------------------------------+
.. versionadded:: 3.1
The ``'surrogateescape'`` and ``'surrogatepass'`` error handlers.
.. versionchanged:: 3.4
The ``'surrogatepass'`` error handler now works with utf-16\* and utf-32\*
codecs.
.. versionadded:: 3.5
The ``'namereplace'`` error handler.
.. versionchanged:: 3.5
The ``'backslashreplace'`` error handler 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:: ignore_errors(exception)
Implements the ``'ignore'`` error handling.
Malformed data is ignored; encoding or decoding is continued without
further notice.
.. function:: replace_errors(exception)
Implements the ``'replace'`` error handling.
Substitutes ``?`` (ASCII character) for encoding errors or ``�`` (U+FFFD,
the official REPLACEMENT CHARACTER) for decoding errors.
.. function:: backslashreplace_errors(exception)
Implements the ``'backslashreplace'`` error handling.
Malformed data is replaced by a backslashed escape sequence.
On encoding, use the hexadecimal form of Unicode code point with formats
:samp:`\\x{hh}` :samp:`\\u{xxxx}` :samp:`\\U{xxxxxxxx}`.
On decoding, use the hexadecimal form of
byte value with format :samp:`\\x{hh}`.
.. versionchanged:: 3.5
Works with decoding and translating.
.. function:: xmlcharrefreplace_errors(exception)
Implements the ``'xmlcharrefreplace'`` error handling (for encoding within
:term:`text encoding` only).
The unencodable character is replaced by an appropriate XML/HTML numeric
character reference, which is a decimal form of Unicode code point with
format :samp:`&#{num};` .
.. function:: namereplace_errors(exception)
Implements the ``'namereplace'`` error handling (for encoding within
:term:`text encoding` only).
The unencodable character is replaced by a ``\N{...}`` escape sequence. The
set of characters that appear in the braces is the Name property from
Unicode Character Database. For example, the German lowercase letter ``'ß'``
will be converted to byte sequence ``\N{LATIN SMALL LETTER SHARP S}`` .
.. 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:
.. class:: Codec
.. method:: encode(input, errors='strict')
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:: decode(input, errors='strict')
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=False)
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=False)
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 iterable of strings to the stream (possibly by reusing
the :meth:`write` method). Infinite or
very large iterables are not supported. 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=-1, chars=-1, firstline=False)
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=None, keepends=True)
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=None, keepends=True)
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:`~StreamReader.read` and :meth:`~StreamWriter.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 ``U+0000``--``U+10FFFF``. (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, for example, you use ``UTF-32-BE`` on a little endian
machine you will always have to swap bytes on encoding and decoding.
Python's ``UTF-16`` and ``UTF-32`` codecs avoid this problem by using the
platform's native byte order when no BOM is present.
Python follows prevailing platform
practice, so native-endian data round-trips without redundant byte swapping,
even though the Unicode Standard defaults to big-endian when the byte order is
unspecified. When these bytes are read by a CPU with a different endianness,
the bytes have to be swapped. To be able to detect the endianness of a
``UTF-16`` or ``UTF-32`` byte sequence, a BOM ("Byte Order Mark") is used.
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.