Fred Drake | b7979c7 | 2000-04-06 14:21:58 +0000 | [diff] [blame^] | 1 | \section{\module{codecs} --- |
| 2 | Python codec registry and base classes} |
| 3 | |
| 4 | \declaremodule{standard}{codec} |
| 5 | \modulesynopsis{Encode and decode data and streams.} |
| 6 | \moduleauthor{Marc-Andre Lemburg}{mal@lemburg.com} |
| 7 | \sectionauthor{Marc-Andre Lemburg}{mal@lemburg.com} |
| 8 | |
| 9 | |
| 10 | \index{Unicode} |
| 11 | \index{Codecs} |
| 12 | \indexii{Codecs}{encode} |
| 13 | \indexii{Codecs}{decode} |
| 14 | \index{streams} |
| 15 | \indexii{stackable}{streams} |
| 16 | |
| 17 | |
| 18 | This module defines base classes for standard Python codecs (encoders |
| 19 | and decoders) and provides access to the internal Python codec |
| 20 | registry which manages the codec lookup process. |
| 21 | |
| 22 | It defines the following functions: |
| 23 | |
| 24 | \begin{funcdesc}{register}{search_function} |
| 25 | Register a codec search function. Search functions are expected to |
| 26 | take one argument, the encoding name in all lower case letters, and |
| 27 | return a tuple of functions \code{(\var{encoder}, \var{decoder}, \var{stream_reader}, |
| 28 | \var{stream_writer})} taking the following arguments: |
| 29 | |
| 30 | \var{encoder} and \var{decoder}: These must be functions or methods |
| 31 | which have the same interface as the .encode/.decode methods of |
| 32 | Codec instances (see Codec Interface). The functions/methods are |
| 33 | expected to work in a stateless mode. |
| 34 | |
| 35 | \var{stream_reader} and \var{stream_writer}: These have to be |
| 36 | factory functions providing the following interface: |
| 37 | |
| 38 | \code{factory(\var{stream},\var{errors}='strict')} |
| 39 | |
| 40 | The factory functions must return objects providing the interfaces |
| 41 | defined by the base classes |
| 42 | \class{StreamWriter}/\class{StreamReader} resp. Stream codecs can |
| 43 | maintain state. |
| 44 | |
| 45 | Possible values for errors are 'strict' (raise an exception in case |
| 46 | of an encoding error), 'replace' (replace malformed data with a |
| 47 | suitable replacement marker, e.g. '?') and 'ignore' (ignore |
| 48 | malformed data and continue without further notice). |
| 49 | |
| 50 | In case a search function cannot find a given encoding, it should |
| 51 | return None. |
| 52 | \end{funcdesc} |
| 53 | |
| 54 | \begin{funcdesc}{lookup}{encoding} |
| 55 | Looks up a codec tuple in the Python codec registry and returns the |
| 56 | function tuple as defined above. |
| 57 | |
| 58 | Encodings are first looked up in the registry's cache. If not found, |
| 59 | the list of registered search functions is scanned. If no codecs tuple |
| 60 | is found, a LookupError is raised. Otherwise, the codecs tuple is |
| 61 | stored in the cache and returned to the caller. |
| 62 | \end{funcdesc} |
| 63 | |
| 64 | To simplify working with encoded files or stream, the module |
| 65 | also defines these utility functions: |
| 66 | |
| 67 | \begin{funcdesc}{open}{filename, mode\optional{, encoding=None, errors='strict', buffering=1}} |
| 68 | Open an encoded file using the given \var{mode} and return |
| 69 | a wrapped version providing transparent encoding/decoding. |
| 70 | |
| 71 | Note: The wrapped version will only accept the object format defined |
| 72 | by the codecs, i.e. Unicode objects for most builtin codecs. Output is |
| 73 | also codec dependent and will usually by Unicode as well. |
| 74 | |
| 75 | \var{encoding} specifies the encoding which is to be used for the |
| 76 | the file. |
| 77 | |
| 78 | \var{errors} may be given to define the error handling. It defaults |
| 79 | to 'strict' which causes a \exception{ValueError} to be raised in case |
| 80 | an encoding error occurs. |
| 81 | |
| 82 | \var{buffering} has the same meaning as for the builtin open() API. |
| 83 | It defaults to line buffered. |
| 84 | \end{funcdesc} |
| 85 | |
| 86 | \begin{funcdesc}{EncodedFile}{file, input\optional{, output=None, errors='strict'}} |
| 87 | |
| 88 | Return a wrapped version of file which provides transparent |
| 89 | encoding translation. |
| 90 | |
| 91 | Strings written to the wrapped file are interpreted according to the |
| 92 | given \var{input} encoding and then written to the original file as |
| 93 | string using the \var{output} encoding. The intermediate encoding will |
| 94 | usually be Unicode but depends on the specified codecs. |
| 95 | |
| 96 | If \var{output} is not given, it defaults to input. |
| 97 | |
| 98 | \var{errors} may be given to define the error handling. It defaults to |
| 99 | 'strict' which causes \exception{ValueError} to be raised in case |
| 100 | an encoding error occurs. |
| 101 | \end{funcdesc} |
| 102 | |
| 103 | |
| 104 | |
| 105 | ...XXX document codec base classes... |
| 106 | |
| 107 | |
| 108 | |
| 109 | The module also provides the following constants which are useful |
| 110 | for reading and writing to platform dependent files: |
| 111 | |
| 112 | \begin{datadesc}{BOM} |
| 113 | \dataline{BOM_BE} |
| 114 | \dataline{BOM_LE} |
| 115 | \dataline{BOM32_BE} |
| 116 | \dataline{BOM32_LE} |
| 117 | \dataline{BOM64_BE} |
| 118 | \dataline{BOM64_LE} |
| 119 | These constants define the byte order marks (BOM) used in data |
| 120 | streams to indicate the byte order used in the stream or file. |
| 121 | \constant{BOM} is either \constant{BOM_BE} or \constant{BOM_LE} |
| 122 | depending on the platform's native byte order, while the others |
| 123 | represent big endian (\samp{_BE} suffix) and little endian |
| 124 | (\samp{_LE} suffix) byte order using 32-bit and 64-bit encodings. |
| 125 | \end{datadesc} |
| 126 | |