4.8 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:

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;
• encoder The stateless encoding function;
• decoder 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:

encoder and decoder: 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 incrementalencoder: 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 IncrementalEncoder, 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 StreamWriter and StreamReader, 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 "?"), 'ignore' (ignore malformed data and continue without further notice), 'xmlcharrefreplace' (replace with the appropriate XML character reference (for encoding only)) and '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.

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:

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.

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.

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.

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.

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.

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.

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.

lookup_error(

name)

Return the error handler previously registered under the name name.

Raises a LookupError in case the handler cannot be found.

strict_errors(

exception)

Implements the strict error handling.

replace_errors(

exception)

Implements the replace error handling.

ignore_errors(

exception)

Implements the ignore error handling.

xmlcharrefreplace_errors_errors(

exception)

Implements the xmlcharrefreplace error handling.

backslashreplace_errors_errors(

exception)

Implements the backslashreplace error handling.

To simplify working with encoded files or stream, the module also defines these utility functions:

open(

filename, mode[, encoding[, errors[, buffering]]])

Open an encoded file using the given mode and return a wrapped version providing transparent encoding/decoding.

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.

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.

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.

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.

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 encoder. New in version 2.5.

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

BOM

BOM_BE

BOM_LE

BOM_UTF8

BOM_UTF16

BOM_UTF16_BE

BOM_UTF16_LE

BOM_UTF32

BOM_UTF32_BE

BOM_UTF32_LE

These constants define various 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.

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Release 2.5, documentation updated on 19th September, 2006.