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Issue 19548: update codecs module documentation

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- clarified the distinction between text encodings and other codecs
- clarified relationship with builtin open and the io module
- consolidated documentation of error handlers into one section
- clarified type constraints of some behaviours
- added tests for some of the new statements in the docs
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Nick Coghlan 2015-01-07 00:22:00 +10:00
parent fcfed19913
commit b9fdb7a452
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Doc/library/codecs.rst
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@ -17,10 +17,17 @@
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.
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`.
It defines the following functions:
The module defines the following functions for encoding and decoding with
any codec:
.. function:: encode(obj, [encoding[, errors]])
@ -28,7 +35,7 @@ It defines the following functions:
encoding is ``utf-8``.
*Errors* may be given to set the desired error handling scheme. The
default error handler is ``strict`` meaning that encoding errors raise
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.
@ -39,90 +46,63 @@ It defines the following functions:
encoding is ``utf-8``.
*Errors* may be given to set the desired error handling scheme. The
default error handler is ``strict`` meaning that decoding errors raise
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:: register(search_function)
Register a codec search function. Search functions are expected to take one
argument, the encoding name in all lower case letters, and return a
:class:`CodecInfo` object having the following attributes:
* ``name`` The name of the encoding;
* ``encode`` The stateless encoding function;
* ``decode`` The stateless decoding function;
* ``incrementalencoder`` An incremental encoder class or factory function;
* ``incrementaldecoder`` An incremental decoder class or factory function;
* ``streamwriter`` A stream writer class or factory function;
* ``streamreader`` A stream reader class or factory function.
The various functions or classes take the following arguments:
*encode* and *decode*: These must be functions or methods which have the same
interface as the :meth:`~Codec.encode`/:meth:`~Codec.decode` methods of Codec
instances (see :ref:`Codec Interface <codec-objects>`). The functions/methods
are expected to work in a stateless mode.
*incrementalencoder* and *incrementaldecoder*: These have to be factory
functions providing the following interface:
``factory(errors='strict')``
The factory functions must return objects providing the interfaces defined by
the base classes :class:`IncrementalEncoder` and :class:`IncrementalDecoder`,
respectively. Incremental codecs can maintain state.
*streamreader* and *streamwriter*: These have to be factory functions providing
the following interface:
``factory(stream, errors='strict')``
The factory functions must return objects providing the interfaces defined by
the base classes :class:`StreamReader` and :class:`StreamWriter`, respectively.
Stream codecs can maintain state.
Possible values for errors are
* ``'strict'``: raise an exception in case of an encoding error
* ``'replace'``: replace malformed data with a suitable replacement marker,
such as ``'?'`` or ``'\ufffd'``
* ``'ignore'``: ignore malformed data and continue without further notice
* ``'xmlcharrefreplace'``: replace with the appropriate XML character
reference (for encoding only)
* ``'backslashreplace'``: replace with backslashed escape sequences (for
encoding only)
* ``'surrogateescape'``: on decoding, replace with code points in the Unicode
Private Use Area ranging from U+DC80 to U+DCFF. These private code
points will then be turned back into the same bytes when the
``surrogateescape`` error handler is used when encoding the data.
(See :pep:`383` for more.)
as well as any other error handling name defined via :func:`register_error`.
In case a search function cannot find a given encoding, it should return
``None``.
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 above.
: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.
To simplify access to the various codecs, the module provides these additional
functions which use :func:`lookup` for the codec lookup:
.. 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)
@ -172,90 +152,43 @@ functions which use :func:`lookup` for the codec lookup:
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_error(name, error_handler)
.. function:: register(search_function)
Register the error handling function *error_handler* under the name *name*.
*error_handler* 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 similar, 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.
.. 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.
.. 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: malformed data is replaced with a
suitable replacement character such as ``'?'`` in bytestrings and
``'\ufffd'`` in Unicode strings.
.. 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 only): the
unencodable character is replaced by an appropriate XML character reference.
.. function:: backslashreplace_errors(exception)
Implements the ``backslashreplace`` error handling (for encoding only): the
unencodable character is replaced by a backslashed escape sequence.
To simplify working with encoded files or stream, the module also defines these
utility functions:
.. function:: open(filename, mode[, encoding[, errors[, buffering]]])
Open an encoded file using the given *mode* and return a wrapped version
providing transparent encoding/decoding. The default file mode is ``'r'``
meaning to open the file in read mode.
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::
The wrapped version's methods will accept and return strings only. Bytes
arguments will be rejected.
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::
Files are always opened in binary mode, even if no binary mode was
specified. This is done to avoid data loss due to encodings using 8-bit
values. This means that no automatic conversion of ``b'\n'`` is done
on reading and writing.
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.
@ -266,12 +199,15 @@ utility functions:
.. function:: EncodedFile(file, data_encoding, file_encoding=None, errors='strict')
Return a wrapped version of file which provides transparent encoding
translation.
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.
Bytes written to the wrapped file are interpreted according to the given
*data_encoding* and then written to the original file as bytes using the
*file_encoding*.
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*.
@ -283,14 +219,16 @@ utility functions:
.. 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`. *errors* (as well as any
*iterator*. This function is a :term:`generator`.
The *errors* argument (as well as any
other keyword argument) is passed through to the incremental encoder.
.. 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`. *errors* (as well as any
*iterator*. This function is a :term:`generator`.
The *errors* argument (as well as any
other keyword argument) is passed through to the incremental decoder.
@ -309,9 +247,10 @@ and writing to platform dependent files:
BOM_UTF32_BE
BOM_UTF32_LE
These constants define various encodings of the Unicode byte order mark (BOM)
used in UTF-16 and UTF-32 data streams to indicate the byte order used in the
stream or file and in UTF-8 as a Unicode signature. :const:`BOM_UTF16` is either
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
@ -325,20 +264,25 @@ Codec Base Classes
------------------
The :mod:`codecs` module defines a set of base classes which define the
interface and can also be used to easily write your own codecs for use in
Python.
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.
implement the file protocols. Codec authors also need to define how the
codec will handle encoding and decoding errors.
The :class:`Codec` class defines the interface for stateless encoders/decoders.
To simplify and standardize error handling, the :meth:`~Codec.encode` and
:meth:`~Codec.decode` methods may implement different error handling schemes by
providing the *errors* string argument. The following string values are defined
and implemented by all standard Python codecs:
.. _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|
@ -346,36 +290,52 @@ and implemented by all standard Python codecs:
| Value | Meaning |
+=========================+===============================================+
| ``'strict'`` | Raise :exc:`UnicodeError` (or a subclass); |
| | this is the default. |
| | this is the default. Implemented in |
| | :func:`strict_errors`. |
+-------------------------+-----------------------------------------------+
| ``'ignore'`` | Ignore the character and continue with the |
| | next. |
| ``'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>`:
+-------------------------+-----------------------------------------------+
| Value | Meaning |
+=========================+===============================================+
| ``'replace'`` | Replace with a suitable replacement |
| | character; Python will use the official |
| | U+FFFD REPLACEMENT CHARACTER for the built-in |
| | Unicode codecs on decoding and '?' on |
| | encoding. |
| | 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). |
| | reference (only for encoding). Implemented |
| | in :func:`xmlcharrefreplace_errors`. |
+-------------------------+-----------------------------------------------+
| ``'backslashreplace'`` | Replace with backslashed escape sequences |
| | (only for encoding). |
| | (only for encoding). Implemented in |
| | :func:`backslashreplace_errors`. |
+-------------------------+-----------------------------------------------+
| ``'surrogateescape'`` | Replace byte with surrogate U+DCxx, as defined|
| | in :pep:`383`. |
| ``'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 handlers are specific to Unicode encoding
schemes:
In addition, the following error handler is specific to the given codecs:
+-------------------+------------------------+-------------------------------------------+
| Value | Codec | Meaning |
| Value | Codecs | Meaning |
+===================+========================+===========================================+
|``'surrogatepass'``| utf-8, utf-16, utf-32, | Allow encoding and decoding of surrogate |
| | utf-16-be, utf-16-le, | codes in all the Unicode encoding schemes.|
| | utf-32-be, utf-32-le | |
| | 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
@ -384,26 +344,96 @@ schemes:
.. versionchanged:: 3.4
The ``'surrogatepass'`` error handlers now works with utf-16\* and utf-32\* codecs.
The set of allowed values can be extended via :meth:`register_error`.
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, ``'<27>'``) 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 encoding with
:term:`text encodings <text encoding>` only): the
unencodable character is replaced by a backslashed escape sequence.
.. _codec-objects:
Codec Objects
^^^^^^^^^^^^^
Stateless Encoding and Decoding
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The :class:`Codec` class defines these methods which also define the function
interfaces of the stateless encoder and decoder:
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).
Encoding converts a string object to a bytes object using a particular
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``).
*errors* defines the error handling to apply. It defaults to ``'strict'``
handling.
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:`StreamCodec` for codecs which have to keep state in order to make
@ -416,14 +446,16 @@ interfaces of the stateless encoder and decoder:
.. method:: Codec.decode(input[, errors])
Decodes the object *input* and returns a tuple (output object, length
consumed). Decoding converts a bytes object encoded using a particular
consumed). For instance, for a :term:`text encoding`, decoding converts
a bytes object encoded using a particular
character set encoding to a string object.
*input* must be a bytes object or one which provides the read-only character
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.
*errors* defines the error handling to apply. It defaults to ``'strict'``
handling.
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:`StreamCodec` for codecs which have to keep state in order to make
@ -432,6 +464,10 @@ interfaces of the stateless encoder and decoder:
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
@ -449,14 +485,14 @@ 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])
.. class:: IncrementalEncoder(errors='strict')
Constructor for an :class:`IncrementalEncoder` instance.
@ -465,26 +501,14 @@ define in order to be compatible with the Python codec registry.
the Python codec registry.
The :class:`IncrementalEncoder` may implement different error handling schemes
by providing the *errors* keyword argument. These parameters are predefined:
* ``'strict'`` Raise :exc:`ValueError` (or a subclass); this is the default.
* ``'ignore'`` Ignore the character and continue with the next.
* ``'replace'`` Replace with a suitable replacement character
* ``'xmlcharrefreplace'`` Replace with the appropriate XML character reference
* ``'backslashreplace'`` Replace with backslashed escape sequences.
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.
The set of allowed values for the *errors* argument can be extended with
:func:`register_error`.
.. method:: encode(object[, final])
@ -496,7 +520,8 @@ define in order to be compatible with the Python codec registry.
.. method:: reset()
Reset the encoder to the initial state. The output is discarded: call
``.encode('', final=True)`` to reset the encoder and to get the output.
``.encode(object, final=True)``, passing an empty byte or text string
if necessary, to reset the encoder and to get the output.
.. method:: IncrementalEncoder.getstate()
@ -517,14 +542,14 @@ define in order to be compatible with the Python codec registry.
.. _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])
.. class:: IncrementalDecoder(errors='strict')
Constructor for an :class:`IncrementalDecoder` instance.
@ -533,22 +558,14 @@ define in order to be compatible with the Python codec registry.
the Python codec registry.
The :class:`IncrementalDecoder` may implement different error handling schemes
by providing the *errors* keyword argument. These parameters are predefined:
* ``'strict'`` Raise :exc:`ValueError` (or a subclass); this is the default.
* ``'ignore'`` Ignore the character and continue with the next.
* ``'replace'`` Replace with a suitable replacement character.
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.
The set of allowed values for the *errors* argument can be extended with
:func:`register_error`.
.. method:: decode(object[, final])
@ -587,6 +604,10 @@ define in order to be compatible with the Python codec registry.
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.
@ -595,14 +616,14 @@ 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])
.. class:: StreamWriter(stream, errors='strict')
Constructor for a :class:`StreamWriter` instance.
@ -610,29 +631,17 @@ compatible with the Python codec registry.
additional keyword arguments, but only the ones defined here are used by the
Python codec registry.
*stream* must be a file-like object open for writing binary data.
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. These parameters are predefined:
* ``'strict'`` Raise :exc:`ValueError` (or a subclass); this is the default.
* ``'ignore'`` Ignore the character and continue with the next.
* ``'replace'`` Replace with a suitable replacement character
* ``'xmlcharrefreplace'`` Replace with the appropriate XML character reference
* ``'backslashreplace'`` Replace with backslashed escape sequences.
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.
The set of allowed values for the *errors* argument can be extended with
:func:`register_error`.
.. method:: write(object)
Writes the object's contents encoded to the stream.
@ -641,7 +650,8 @@ compatible with the Python codec registry.
.. method:: writelines(list)
Writes the concatenated list of strings to the stream (possibly by reusing
the :meth:`write` method).
the :meth:`write` method). The standard bytes-to-bytes codecs
do not support this method.
.. method:: reset()
@ -660,14 +670,14 @@ 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])
.. class:: StreamReader(stream, errors='strict')
Constructor for a :class:`StreamReader` instance.
@ -675,16 +685,12 @@ compatible with the Python codec registry.
additional keyword arguments, but only the ones defined here are used by the
Python codec registry.
*stream* must be a file-like object open for reading (binary) data.
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. These parameters are defined:
* ``'strict'`` Raise :exc:`ValueError` (or a subclass); this is the default.
* ``'ignore'`` Ignore the character and continue with the next.
* ``'replace'`` Replace with a suitable replacement character.
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
@ -698,17 +704,20 @@ compatible with the Python codec registry.
Decodes data from the stream and returns the resulting object.
*chars* indicates the number of characters to read from the
stream. :func:`read` will never return more than *chars* characters, but
it might return less, if there are not enough characters available.
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.
*size* indicates the approximate maximum number of bytes to read from the
stream for decoding purposes. The decoder can modify this setting as
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. *size* is intended to prevent having to decode huge files in
one step.
possible. This parameter is intended to
prevent having to decode huge files in one step.
*firstline* indicates that it would be sufficient to only return the first
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
@ -751,17 +760,13 @@ compatible with the Python codec registry.
In addition to the above methods, the :class:`StreamReader` must also inherit
all other methods and attributes from the underlying stream.
The next two base classes are included for convenience. They are not needed by
the codec registry, but may provide useful in practice.
.. _stream-reader-writer:
StreamReaderWriter Objects
^^^^^^^^^^^^^^^^^^^^^^^^^^
~~~~~~~~~~~~~~~~~~~~~~~~~~
The :class:`StreamReaderWriter` allows wrapping streams which work in both read
and write modes.
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.
@ -782,9 +787,9 @@ methods and attributes from the underlying stream.
.. _stream-recoder-objects:
StreamRecoder Objects
^^^^^^^^^^^^^^^^^^^^^
~~~~~~~~~~~~~~~~~~~~~
The :class:`StreamRecoder` provide a frontend - backend view of encoding data
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
@ -794,22 +799,20 @@ The design is such that one can use the factory functions returned by the
.. class:: StreamRecoder(stream, encode, decode, Reader, Writer, errors)
Creates a :class:`StreamRecoder` instance which implements a two-way conversion:
*encode* and *decode* work on the frontend (the input to :meth:`read` and output
of :meth:`write`) while *Reader* and *Writer* work on the backend (reading and
writing to the stream).
*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 direct recodings from e.g. Latin-1
You can use these objects to do transparent transcodings from e.g. Latin-1
to UTF-8 and back.
*stream* must be a file-like object.
The *stream* argument must be a file-like object.
*encode*, *decode* must adhere to the :class:`Codec` interface. *Reader*,
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.
*encode* and *decode* are needed for the frontend translation, *Reader* and
*Writer* for the backend translation.
Error handling is done in the same way as defined for the stream readers and
writers.
@ -824,20 +827,23 @@ methods and attributes from the underlying stream.
Encodings and Unicode
---------------------
Strings are stored internally as sequences of codepoints in range ``0 - 10FFFF``
(see :pep:`393` for more details about the implementation).
Once a string object is used outside of CPU and memory, CPU endianness
and how these arrays are stored as bytes become an issue. Transforming a
string object into a sequence of bytes is called encoding and recreating the
string object from the sequence of bytes is known as decoding. There are many
different methods for how this transformation can be done (these methods are
also called encodings). The simplest method is to map the codepoints 0-255 to
the bytes ``0x0``-``0xff``. This means that a string object that contains
codepoints above ``U+00FF`` can't be encoded with this method (which is called
``'latin-1'`` or ``'iso-8859-1'``). :func:`str.encode` will raise a
:exc:`UnicodeEncodeError` that looks like this: ``UnicodeEncodeError: 'latin-1'
codec can't encode character '\u1234' in position 3: ordinal not in
range(256)``.
Strings are stored internally as sequences of codepoints 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 codepoints 0-255 to the bytes ``0x0``-``0xff``, which means that a string
object that contains codepoints 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 codepoints are
@ -1184,7 +1190,8 @@ particular, the following variants typically exist:
.. 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
(``U+D800``--``U+DFFF``) to be encoded.
The utf-32\* decoders no longer decode
byte sequences that correspond to surrogate code points.
@ -1212,7 +1219,9 @@ encodings.
+====================+=========+===========================+
| idna | | Implements :rfc:`3490`, |
| | | see also |
| | | :mod:`encodings.idna` |
| | | :mod:`encodings.idna`. |
| | | Only ``errors='strict'`` |
| | | is supported. |
+--------------------+---------+---------------------------+
| mbcs | dbcs | Windows only: Encode |
| | | operand according to the |
@ -1220,31 +1229,44 @@ encodings.
+--------------------+---------+---------------------------+
| palmos | | Encoding of PalmOS 3.5 |
+--------------------+---------+---------------------------+
| punycode | | Implements :rfc:`3492` |
| punycode | | Implements :rfc:`3492`. |
| | | Stateful codecs are not |
| | | supported. |
+--------------------+---------+---------------------------+
| raw_unicode_escape | | Produce a string that is |
| | | suitable as raw Unicode |
| | | literal in Python source |
| | | code |
| 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. Can be |
| | | used as the system |
| | | encoding if no automatic |
| | | coercion between byte and |
| | | Unicode strings is |
| | | desired. |
| | | all conversions, even |
| | | empty strings. The error |
| | | handler is ignored. |
+--------------------+---------+---------------------------+
| unicode_escape | | Produce a string that is |
| | | suitable as Unicode |
| | | literal in Python source |
| | | code |
| 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 |
| | | operand. Stateful codecs |
| | | are not supported. |
| | | |
| | | .. deprecated:: 3.3 |
| | | This representation is |
| | | obsoleted by |
| | | :pep:`393`. |
+--------------------+---------+---------------------------+
.. _binary-transforms:
@ -1253,7 +1275,8 @@ Binary Transforms
^^^^^^^^^^^^^^^^^
The following codecs provide binary transforms: :term:`bytes-like object`
to :class:`bytes` mappings.
to :class:`bytes` mappings. They are not supported by :meth:`bytes.decode`
(which only produces :class:`str` output).
.. tabularcolumns:: |l|L|L|L|
@ -1308,7 +1331,8 @@ Text Transforms
^^^^^^^^^^^^^^^
The following codec provides a text transform: a :class:`str` to :class:`str`
mapping.
mapping. It is not supported by :meth:`str.encode` (which only produces
:class:`bytes` output).
.. tabularcolumns:: |l|l|L|

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@ -939,15 +939,17 @@ are always available. They are listed here in alphabetical order.
*encoding* is the name of the encoding used to decode or encode the file.
This should only be used in text mode. The default encoding is platform
dependent (whatever :func:`locale.getpreferredencoding` returns), but any
encoding supported by Python can be used. See the :mod:`codecs` module for
:term:`text encoding` supported by Python
can be used. See the :mod:`codecs` module for
the list of supported encodings.
*errors* is an optional string that specifies how encoding and decoding
errors are to be handled--this cannot be used in binary mode.
A variety of standard error handlers are available, though any
A variety of standard error handlers are available
(listed under :ref:`error-handlers`), though any
error handling name that has been registered with
:func:`codecs.register_error` is also valid. The standard names
are:
include:
* ``'strict'`` to raise a :exc:`ValueError` exception if there is
an encoding error. The default value of ``None`` has the same

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@ -1512,7 +1512,7 @@ expression support in the :mod:`re` module).
a :exc:`UnicodeError`. Other possible
values are ``'ignore'``, ``'replace'``, ``'xmlcharrefreplace'``,
``'backslashreplace'`` and any other name registered via
:func:`codecs.register_error`, see section :ref:`codec-base-classes`. For a
:func:`codecs.register_error`, see section :ref:`error-handlers`. For a
list of possible encodings, see section :ref:`standard-encodings`.
.. versionchanged:: 3.1
@ -2384,7 +2384,7 @@ arbitrary binary data.
error handling scheme. The default for *errors* is ``'strict'``, meaning
that encoding errors raise a :exc:`UnicodeError`. Other possible values are
``'ignore'``, ``'replace'`` and any other name registered via
:func:`codecs.register_error`, see section :ref:`codec-base-classes`. For a
:func:`codecs.register_error`, see section :ref:`error-handlers`. For a
list of possible encodings, see section :ref:`standard-encodings`.
.. note::

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@ -794,7 +794,7 @@ metadata must be either decoded or encoded. If *encoding* is not set
appropriately, this conversion may fail.
The *errors* argument defines how characters are treated that cannot be
converted. Possible values are listed in section :ref:`codec-base-classes`.
converted. Possible values are listed in section :ref:`error-handlers`.
The default scheme is ``'surrogateescape'`` which Python also uses for its
file system calls, see :ref:`os-filenames`.