# 4.9 `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 tuple of functions `(encoder, decoder, stream_reader, stream_writer)` taking 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.

  stream_reader and stream_writer: 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 a codec tuple in the Python codec registry and returns the function tuple 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 codecs tuple is found, a `LookupError` is raised. Otherwise, the codecs tuple 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)

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

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

`getdecoder`( | encoding)

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

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

`getreader`( | encoding)

  Lookup 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)

  Lookup 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 register 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.

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.