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7.8. codecs — Codec registry and base classes

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.

It defines the following functions:

codecs. encode ( obj [, encoding [, errors ] ] )

Encodes obj using the codec registered for encoding. The default encoding is 'ascii'.

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.

New in version 2.4.

codecs. decode ( obj [, encoding [, errors ] ] )

Decodes obj using the codec registered for encoding. The default encoding is 'ascii'.

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.

New in version 2.4.

codecs. 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 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 encode()/decode() methods of Codec instances (see Codec Interface). 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 IncrementalEncoder and 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 StreamReader and 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)

as well as any other error handling name defined via register_error().

In case a search function cannot find a given encoding, it should return None.

codecs. lookup ( encoding )

Looks up the codec info in the Python codec registry and returns a CodecInfo object as defined above.

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.

To simplify access to the various codecs, 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.

New in version 2.5.

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.

New in version 2.5.

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.

codecs. register_error ( name, error_handler )

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 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 encoder will encode the replacement and continue encoding the 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 similar, 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.

codecs. lookup_error ( name )

Return the error handler previously registered under the name name.

Raises a LookupError in case the handler cannot be found.

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: malformed data is replaced with a suitable replacement character such as '?' in bytestrings and '\ufffd' in Unicode strings.

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 only): the unencodable character is replaced by an appropriate XML character reference.

codecs. 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:

codecs. 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.

Note

The wrapped version will only accept the object format defined by the codecs, i.e. Unicode objects for most built-in codecs. Output is also codec-dependent and will usually be Unicode as well.

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 '\n' is done on reading and writing.

encoding specifies the encoding which is to be used for the file.

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, input [, output [, errors ] ] )

Return a wrapped version of file which provides transparent encoding translation.

Strings written to the wrapped file are interpreted according to the given input encoding and then written to the original file as strings using the output encoding. The intermediate encoding will usually be Unicode but depends on the specified codecs.

If output is not given, it defaults to input.

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 ( iterable, encoding [, errors ] )

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

New in version 2.5.

codecs. iterdecode ( iterable, encoding [, errors ] )

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

New in version 2.5.

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 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. 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.

7.8.1. Codec Base Classes

The 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.

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.

The Codec class defines the interface for stateless encoders/decoders.

To simplify and standardize error handling, the encode() and 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:

Value

Meaning

'strict'

Raise UnicodeError (or a subclass); this is the default.

'ignore'

Ignore the character and continue with the next.

'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.

'xmlcharrefreplace'

Replace with the appropriate XML character reference (only for encoding).

'backslashreplace'

Replace with backslashed escape sequences (only for encoding).

The set of allowed values can be extended via register_error().

7.8.1.1. Codec Objects

The 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). While codecs are not restricted to use with Unicode, in a Unicode context, encoding converts a Unicode object to a plain string 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 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). In a Unicode context, decoding converts a plain string encoded using a particular character set encoding to a Unicode object.

input must be an object which provides the bf_getreadbuf buffer slot. Python strings, buffer objects and memory mapped files are examples of objects providing this slot.

errors 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.

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.

7.8.1.2. IncrementalEncoder Objects

New in version 2.5.

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 ] )

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. These parameters are predefined:

  • 'strict' Raise 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.

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.

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

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.

7.8.1.3. 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 ] )

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. These parameters are predefined:

  • 'strict' Raise ValueError (or a subclass); this is the default.

  • 'ignore' Ignore the character and continue with the next.

  • 'replace' Replace with a suitable replacement character.

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 IncrementalDecoder object.

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

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.

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.

7.8.1.4. 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 ] )

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.

stream must be a file-like object open for writing binary data.

The StreamWriter may implement different error handling schemes by providing the errors keyword argument. These parameters are predefined:

  • 'strict' Raise 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.

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 StreamWriter object.

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

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).

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.

7.8.1.5. 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 ] )

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.

stream must be a file-like object open for reading (binary) data.

The StreamReader may implement different error handling schemes by providing the errors keyword argument. These parameters are defined:

  • 'strict' Raise ValueError (or a subclass); this is the default.

  • 'ignore' Ignore the character and continue with the next.

  • 'replace' Replace with a suitable replacement character.

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 StreamReader object.

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.

chars indicates the number of characters to read from the stream. read() will never return more than chars characters, but it might return less, if there are not enough characters available.

size indicates the approximate maximum number of bytes to read from the stream for decoding purposes. 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.

firstline 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.

Changed in version 2.4: chars argument added.

Changed in version 2.4.2: firstline argument added.

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.

Changed in version 2.4: keepends argument added.

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.

The next two base classes are included for convenience. They are not needed by the codec registry, but may provide useful in practice.

7.8.1.6. StreamReaderWriter Objects

The StreamReaderWriter 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 )

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.

7.8.1.7. StreamRecoder Objects

The StreamRecoder provide a frontend - backend view of encoding data 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 )

Creates a StreamRecoder instance which implements a two-way conversion: encode and decode work on the frontend (the input to read() and output of write()) while Reader and Writer work on the backend (reading and writing to the stream).

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

stream must be a file-like object.

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

encode and decode are needed for the frontend translation, Reader and Writer for the backend translation. The intermediate format used is determined by the two sets of codecs, e.g. the Unicode codecs will use Unicode as the intermediate encoding.

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.

7.8.2. Encodings and Unicode

Unicode strings are stored internally as sequences of code points (to be precise as Py_UNICODE arrays). Depending on the way Python is compiled (either via --enable-unicode=ucs2 or --enable-unicode=ucs4, with the former being the default) Py_UNICODE is either a 16-bit or 32-bit data type. Once a Unicode object is used outside of CPU and memory, CPU endianness and how these arrays are stored as bytes become an issue. Transforming a unicode object into a sequence of bytes is called encoding and recreating the unicode 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 code points 0–255 to the bytes 0x00xff. This means that a unicode object that contains code points above U+00FF can’t be encoded with this method (which is called 'latin-1' or 'iso-8859-1'). unicode.encode() will raise a UnicodeEncodeError that looks like this: UnicodeEncodeError: 'latin-1' codec can't encode character u'\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 0x00xff. 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 Unicode 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-00000000U-0000007F

0xxxxxxx

U-00000080U-000007FF

110xxxxx 10xxxxxx

U-00000800U-0000FFFF

1110xxxx 10xxxxxx 10xxxxxx

U-00010000U-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 Unicode 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 Unicode 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.

7.8.3. 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.

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

cp424

EBCDIC-CP-HE, IBM424

Hebrew

cp437

437, IBM437

English

cp500

EBCDIC-CP-BE, EBCDIC-CP-CH, IBM500

Western Europe

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

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

euc_jp

eucjp, ujis, u-jis

Japanese

euc_jis_2004

jisx0213, eucjis2004

Japanese

euc_jisx0213

eucjisx0213

Japanese

euc_kr

euckr, korean, ksc5601, ks_c-5601, ks_c-5601-1987, ksx1001, ks_x-1001

Korean

gb2312

chinese, csiso58gb231280, euc- cn, euccn, eucgb2312-cn, gb2312-1980, gb2312-80, iso- ir-58

Simplified Chinese

gbk

936, cp936, ms936

Unified Chinese

gb18030

gb18030-2000

Unified Chinese

hz

hzgb, hz-gb, hz-gb-2312

Simplified Chinese

iso2022_jp

csiso2022jp, iso2022jp, iso-2022-jp

Japanese

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, iso-2022-kr

Korean

latin_1

iso-8859-1, iso8859-1, 8859, cp819, latin, latin1, L1

West Europe

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_u

Ukrainian

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

Western Europe

mac_turkish

macturkish

Turkish

ptcp154

csptcp154, pt154, cp154, cyrillic-asian

Kazakh

shift_jis

csshiftjis, shiftjis, sjis, s_jis

Japanese

shift_jis_2004

shiftjis2004, sjis_2004, sjis2004

Japanese

shift_jisx0213

shiftjisx0213, sjisx0213, s_jisx0213

Japanese

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 (BMP only)

utf_16_le

UTF-16LE

all languages (BMP only)

utf_7

U7, unicode-1-1-utf-7

all languages

utf_8

U8, UTF, utf8

all languages

utf_8_sig

all languages

7.8.4. 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.

The following codecs provide unicode-to-str encoding 1 and str-to-unicode decoding 2, similar to the Unicode text encodings.

Codec

Aliases

Purpose

idna

Implements RFC 3490 , see also encodings.idna

mbcs

dbcs

Windows only: Encode operand according to the ANSI codepage (CP_ACP)

palmos

Encoding of PalmOS 3.5

punycode

Implements RFC 3492

raw_unicode_escape

Produce a string that is suitable as raw Unicode literal in Python source code

rot_13

rot13

Returns the Caesar-cypher encryption of the operand

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.

unicode_escape

Produce a string that is suitable as Unicode literal in Python source code

unicode_internal

Return the internal representation of the operand

New in version 2.3: The idna and punycode encodings.

The following codecs provide str-to-str encoding and decoding 2.

Codec

Aliases

Purpose

Encoder/decoder

base64_codec

base64, base-64

Convert operand to multiline MIME base64 (the result always includes a trailing '\n')

base64.encodestring(), base64.decodestring()

bz2_codec

bz2

Compress the operand using bz2

bz2.compress(), bz2.decompress()

hex_codec

hex

Convert operand to hexadecimal representation, with two digits per byte

binascii.b2a_hex(), binascii.a2b_hex()

quopri_codec

quopri, quoted-printable, quotedprintable

Convert operand to MIME quoted printable

quopri.encode() with quotetabs=True, quopri.decode()

string_escape

Produce a string that is suitable as string literal in Python source code

uu_codec

uu

Convert the operand using uuencode

uu.encode(), uu.decode()

zlib_codec

zip, zlib

Compress the operand using gzip

zlib.compress(), zlib.decompress()

1

str objects are also accepted as input in place of unicode objects. They are implicitly converted to unicode by decoding them using the default encoding. If this conversion fails, it may lead to encoding operations raising UnicodeDecodeError.

2 (1,2)

unicode objects are also accepted as input in place of str objects. They are implicitly converted to str by encoding them using the default encoding. If this conversion fails, it may lead to decoding operations raising UnicodeEncodeError.

7.8.5. encodings.idna — Internationalized Domain Names in Applications

New in version 2.3.

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 httplib and ftplib, accept Unicode host names (httplib 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 .

7.8.6. encodings.utf_8_sig — UTF-8 codec with BOM signature

New in version 2.5.

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.