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Python 3.6.5 Documentation > "codecs" — Codec registry and base classes

"codecs" — Codec registry and base classes
******************************************

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

======================================================================

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 *text encodings*, 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 *text
encodings*, or with codecs that encode to "bytes".

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

codecs.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 "ValueError" (or a more codec specific subclass, such as
"UnicodeEncodeError"). Refer to Codec Base Classes for more
information on codec error handling.

codecs.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 "ValueError" (or a more codec specific subclass, such as
"UnicodeDecodeError"). Refer to Codec Base Classes for more
information on codec error handling.

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

codecs.lookup(encoding)

Looks up the codec info in the Python codec registry and returns a
"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
"CodecInfo" object is found, a "LookupError" is raised. Otherwise,
the "CodecInfo" object is stored in the cache and returned to the
caller.

class codecs.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:

name

The name of the encoding.

encode
decode

The stateless encoding and decoding functions. These must be
functions or methods which have the same interface as the
"encode()" and "decode()" methods of Codec instances (see Codec
Interface). The functions or methods are expected to work in a
stateless mode.

incrementalencoder
incrementaldecoder

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

streamwriter
streamreader

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

To simplify access to the various codec components, the module
provides these additional functions which use "lookup()" for the codec
lookup:

codecs.getencoder(encoding)

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

Raises a "LookupError" in case the encoding cannot be found.

codecs.getdecoder(encoding)

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

Raises a "LookupError" in case the encoding cannot be found.

codecs.getincrementalencoder(encoding)

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

Raises a "LookupError" in case the encoding cannot be found or the
codec doesn’t support an incremental encoder.

codecs.getincrementaldecoder(encoding)

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

Raises a "LookupError" in case the encoding cannot be found or the
codec doesn’t support an incremental decoder.

codecs.getreader(encoding)

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

Raises a "LookupError" in case the encoding cannot be found.

codecs.getwriter(encoding)

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

Raises a "LookupError" in case the encoding cannot be found.

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

codecs.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 "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 "open()" and the associated "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:

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

Open an encoded file using the given *mode* and return an instance
of "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 "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 "ValueError" to be raised in case an
encoding error occurs.

*buffering* has the same meaning as for the built-in "open()"
function. It defaults to line buffered.

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

Return a "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 "ValueError" to be raised in case an
encoding error occurs.

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

Uses an incremental encoder to iteratively encode the input
provided by *iterator*. This function is a *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 "str" objects to
encode. Therefore it does not support bytes-to-bytes encoders such
as "base64_codec".

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

Uses an incremental decoder to iteratively decode the input
provided by *iterator*. This function is a *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 "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
"iterencode()".

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

codecs.BOM
codecs.BOM_BE
codecs.BOM_LE
codecs.BOM_UTF8
codecs.BOM_UTF16
codecs.BOM_UTF16_BE
codecs.BOM_UTF16_LE
codecs.BOM_UTF32
codecs.BOM_UTF32_BE
codecs.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. "BOM_UTF16" is either "BOM_UTF16_BE"
or "BOM_UTF16_LE" depending on the platform’s native byte order,
"BOM" is an alias for "BOM_UTF16", "BOM_LE" for "BOM_UTF16_LE" and
"BOM_BE" for "BOM_UTF16_BE". The others represent the BOM in UTF-8
and UTF-32 encodings.

Codec Base Classes
==================

The "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.

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:

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

The following error handlers are only applicable to *text encodings*:

+---------------------------+-------------------------------------------------+
| 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 "replace_errors()". |
+---------------------------+-------------------------------------------------+
| "'xmlcharrefreplace'" | Replace with the appropriate XML character |
| | reference (only for encoding). Implemented in |
| | "xmlcharrefreplace_errors()". |
+---------------------------+-------------------------------------------------+
| "'backslashreplace'" | Replace with backslashed escape sequences. |
| | Implemented in "backslashreplace_errors()". |
+---------------------------+-------------------------------------------------+
| "'namereplace'" | Replace with "\N{...}" escape sequences (only |
| | for encoding). Implemented in |
| | "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:

New in version 3.1: The "'surrogateescape'" and "'surrogatepass'"
error handlers.

Changed in version 3.4: The "'surrogatepass'" error handlers now works
with utf-16* and utf-32* codecs.

New in version 3.5: The "'namereplace'" error handler.

Changed in version 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:

codecs.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
"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 "str" or "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 "IndexError" will be raised.

Decoding and translating works similarly, except
"UnicodeDecodeError" or "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:

codecs.lookup_error(name)

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

Raises a "LookupError" in case the handler cannot be found.

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

codecs.strict_errors(exception)

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

codecs.replace_errors(exception)

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

codecs.ignore_errors(exception)

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

codecs.xmlcharrefreplace_errors(exception)

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

codecs.backslashreplace_errors(exception)

Implements the "'backslashreplace'" error handling (for *text
encodings* only): malformed data is replaced by a backslashed
escape sequence.

codecs.namereplace_errors(exception)

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

New in version 3.5.

Stateless Encoding and Decoding
-------------------------------

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

Codec.encode(input[, errors])

Encodes the object *input* and returns a tuple (output object,
length consumed). For instance, *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 "Codec" instance. Use
"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.

Codec.decode(input[, errors])

Decodes the object *input* and returns a tuple (output object,
length consumed). For instance, for a *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 "Codec" instance. Use
"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 "IncrementalEncoder" and "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
"encode()"/"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 "encode()"/"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.

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

The "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 codecs.IncrementalEncoder(errors='strict')

Constructor for an "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 "IncrementalEncoder" may implement different error handling
schemes by providing the *errors* keyword argument. See 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 "IncrementalEncoder" object.

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 "encode()" *final* must be true (the default is
false).

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.

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).

setstate(state)

Set the state of the encoder to *state*. *state* must be an
encoder state returned by "getstate()".

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

The "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 codecs.IncrementalDecoder(errors='strict')

Constructor for an "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 "IncrementalDecoder" may implement different error handling
schemes by providing the *errors* keyword argument. See Error
Handlers for possible values.

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 "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).

reset()

Reset the decoder to the initial state.

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.)

setstate(state)

Set the state of the encoder to *state*. *state* must be a
decoder state returned by "getstate()".

Stream Encoding and Decoding
----------------------------

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

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

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

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

Constructor for a "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 "StreamWriter" may implement different error handling schemes
by providing the *errors* keyword argument. See Error Handlers for
the standard error handlers the underlying stream codec may
support.

write(object)

Writes the object’s contents encoded to the stream.

writelines(list)

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

reset()

Flushes and resets the codec buffers used for keeping state.

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

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

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

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

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

Constructor for a "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 "StreamReader" may implement different error handling schemes
by providing the *errors* keyword argument. See Error Handlers for
the standard error handlers the underlying stream codec may
support.

The set of allowed values for the *errors* argument can be extended
with "register_error()".

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 "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.

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
"read()" method.

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

readlines([sizehint[, keepends]])

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

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

*sizehint*, if given, is passed as the *size* argument to the
stream’s "read()" method.

reset()

Resets the codec buffers used for keeping state.

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

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

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

The "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 "lookup()" function to construct the instance.

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

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

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

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

The "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 "lookup()" function to construct the instance.

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

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

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

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

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

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

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

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 *text encodings*.

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 "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. "encodings/cp1252.py" (which is an
encoding that is used primarily on Windows). There’s a string constant
with 256 characters that shows you which character is mapped to which
byte value.

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

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

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

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

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

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

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

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

Standard Encodings
==================

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

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

Changed in version 3.6: Optimization opportunity recognized for us-
ascii.

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

* an ISO 8859 codeset

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

* an IBM EBCDIC code page

* an IBM PC code page, which is ASCII compatible

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

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

Python Specific Encodings
=========================

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

Text Encodings
--------------

The following codecs provide "str" to "bytes" encoding and *bytes-like
object* to "str" decoding, similar to the Unicode text encodings.

Binary Transforms
-----------------

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

[1] In addition to *bytes-like objects*, "'base64_codec'" also
accepts ASCII-only instances of "str" for decoding

New in version 3.2: Restoration of the binary transforms.

Changed in version 3.4: Restoration of the aliases for the binary
transforms.

Text Transforms
---------------

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

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

New in version 3.2: Restoration of the "rot_13" text transform.

Changed in version 3.4: Restoration of the "rot13" alias.

"encodings.idna" — Internationalized Domain Names in Applications
=================================================================

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

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

Python supports this conversion in several ways: the "idna" codec
performs conversion between Unicode and ACE, separating an input
string into labels based on the separator characters defined in
section 3.1 (1) of **RFC 3490** and converting each label to ACE as
required, and conversely separating an input byte string into labels
based on the "." separator and converting any ACE labels found into
unicode. Furthermore, the "socket" module transparently converts
Unicode host names to ACE, so that applications need not be concerned
about converting host names themselves when they pass them to the
socket module. On top of that, modules that have host names as
function parameters, such as "http.client" and "ftplib", accept
Unicode host names ("http.client" then also transparently sends an
IDNA hostname in the *Host* field if it sends that field at all).

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

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

encodings.idna.nameprep(label)

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

encodings.idna.ToASCII(label)

Convert a label to ASCII, as specified in **RFC 3490**.
"UseSTD3ASCIIRules" is assumed to be false.

encodings.idna.ToUnicode(label)

Convert a label to Unicode, as specified in **RFC 3490**.

"encodings.mbcs" — Windows ANSI codepage
========================================

Encode operand according to the ANSI codepage (CP_ACP).

Availability: Windows only.

Changed in version 3.3: Support any error handler.

Changed in version 3.2: Before 3.2, the *errors* argument was ignored;
"'replace'" was always used to encode, and "'ignore'" to decode.

"encodings.utf_8_sig" — UTF-8 codec with BOM signature
======================================================

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

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