484 lines
18 KiB
TeX
484 lines
18 KiB
TeX
\section{\module{codecs} ---
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Codec registry and base classes}
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\declaremodule{standard}{codecs}
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\modulesynopsis{Encode and decode data and streams.}
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\moduleauthor{Marc-Andre Lemburg}{mal@lemburg.com}
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\sectionauthor{Marc-Andre Lemburg}{mal@lemburg.com}
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\index{Unicode}
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\index{Codecs}
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\indexii{Codecs}{encode}
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\indexii{Codecs}{decode}
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\index{streams}
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\indexii{stackable}{streams}
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This module defines base classes for standard Python codecs (encoders
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and decoders) and provides access to the internal Python codec
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registry which manages the codec and error handling lookup process.
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It defines the following functions:
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\begin{funcdesc}{register}{search_function}
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Register a codec search function. Search functions are expected to
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take one argument, the encoding name in all lower case letters, and
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return a tuple of functions \code{(\var{encoder}, \var{decoder}, \var{stream_reader},
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\var{stream_writer})} taking the following arguments:
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\var{encoder} and \var{decoder}: These must be functions or methods
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which have the same interface as the
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\method{encode()}/\method{decode()} methods of Codec instances (see
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Codec Interface). The functions/methods are expected to work in a
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stateless mode.
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\var{stream_reader} and \var{stream_writer}: These have to be
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factory functions providing the following interface:
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\code{factory(\var{stream}, \var{errors}='strict')}
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The factory functions must return objects providing the interfaces
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defined by the base classes \class{StreamWriter} and
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\class{StreamReader}, respectively. Stream codecs can maintain
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state.
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Possible values for errors are \code{'strict'} (raise an exception
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in case of an encoding error), \code{'replace'} (replace malformed
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data with a suitable replacement marker, such as \character{?}) and
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\code{'ignore'} (ignore malformed data and continue without further
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notice).
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In case a search function cannot find a given encoding, it should
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return \code{None}.
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\end{funcdesc}
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\begin{funcdesc}{lookup}{encoding}
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Looks up a codec tuple in the Python codec registry and returns the
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function tuple as defined above.
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Encodings are first looked up in the registry's cache. If not found,
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the list of registered search functions is scanned. If no codecs tuple
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is found, a \exception{LookupError} is raised. Otherwise, the codecs
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tuple is stored in the cache and returned to the caller.
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\end{funcdesc}
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To simplify access to the various codecs, the module provides these
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additional functions which use \function{lookup()} for the codec
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lookup:
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\begin{funcdesc}{getencoder}{encoding}
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Lookup up the codec for the given encoding and return its encoder
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function.
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Raises a \exception{LookupError} in case the encoding cannot be found.
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\end{funcdesc}
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\begin{funcdesc}{getdecoder}{encoding}
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Lookup up the codec for the given encoding and return its decoder
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function.
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Raises a \exception{LookupError} in case the encoding cannot be found.
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\end{funcdesc}
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\begin{funcdesc}{getreader}{encoding}
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Lookup up the codec for the given encoding and return its StreamReader
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class or factory function.
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Raises a \exception{LookupError} in case the encoding cannot be found.
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\end{funcdesc}
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\begin{funcdesc}{getwriter}{encoding}
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Lookup up the codec for the given encoding and return its StreamWriter
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class or factory function.
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Raises a \exception{LookupError} in case the encoding cannot be found.
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\end{funcdesc}
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To simplify working with encoded files or stream, the module
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also defines these utility functions:
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\begin{funcdesc}{register_error}{name, error_handler}
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Register the error handling function \var{error_handler} under the
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name \var{name}. \vari{error_handler} will be called during encoding
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and decoding in case of an error, when \var{name} is specified as the
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errors parameter. \var{error_handler} will be called with an
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\exception{UnicodeEncodeError}, \exception{UnicodeDecodeError} or
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\exception{UnicodeTranslateError} instance and must return a tuple
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with a replacement for the unencodable/undecodable part of the input
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and a position where encoding/decoding should continue.
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\end{funcdesc}
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\begin{funcdesc}{lookup_error}{name}
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Return the error handler previously register under the name \var{name}.
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Raises a \exception{LookupError} in case the handler cannot be found.
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\end{funcdesc}
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\begin{funcdesc}{strict_errors}{exception}
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Implements the \code{strict} error handling.
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\end{funcdesc}
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\begin{funcdesc}{replace_errors}{exception}
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Implements the \code{replace} error handling.
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\end{funcdesc}
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\begin{funcdesc}{ignore_errors}{exception}
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Implements the \code{ignore} error handling.
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\end{funcdesc}
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\begin{funcdesc}{xmlcharrefreplace_errors_errors}{exception}
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Implements the \code{xmlcharrefreplace} error handling.
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\end{funcdesc}
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\begin{funcdesc}{backslashreplace_errors_errors}{exception}
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Implements the \code{backslashreplace} error handling.
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\end{funcdesc}
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\begin{funcdesc}{open}{filename, mode\optional{, encoding\optional{,
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errors\optional{, buffering}}}}
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Open an encoded file using the given \var{mode} and return
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a wrapped version providing transparent encoding/decoding.
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\note{The wrapped version will only accept the object format
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defined by the codecs, i.e.\ Unicode objects for most built-in
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codecs. Output is also codec-dependent and will usually be Unicode as
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well.}
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\var{encoding} specifies the encoding which is to be used for the
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the file.
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\var{errors} may be given to define the error handling. It defaults
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to \code{'strict'} which causes a \exception{ValueError} to be raised
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in case an encoding error occurs.
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\var{buffering} has the same meaning as for the built-in
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\function{open()} function. It defaults to line buffered.
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\end{funcdesc}
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\begin{funcdesc}{EncodedFile}{file, input\optional{,
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output\optional{, errors}}}
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Return a wrapped version of file which provides transparent
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encoding translation.
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Strings written to the wrapped file are interpreted according to the
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given \var{input} encoding and then written to the original file as
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strings using the \var{output} encoding. The intermediate encoding will
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usually be Unicode but depends on the specified codecs.
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If \var{output} is not given, it defaults to \var{input}.
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\var{errors} may be given to define the error handling. It defaults to
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\code{'strict'}, which causes \exception{ValueError} to be raised in case
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an encoding error occurs.
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\end{funcdesc}
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The module also provides the following constants which are useful
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for reading and writing to platform dependent files:
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\begin{datadesc}{BOM}
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\dataline{BOM_BE}
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\dataline{BOM_LE}
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\dataline{BOM_UTF8}
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\dataline{BOM_UTF16}
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\dataline{BOM_UTF16_BE}
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\dataline{BOM_UTF16_LE}
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\dataline{BOM_UTF32}
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\dataline{BOM_UTF32_BE}
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\dataline{BOM_UTF32_LE}
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These constants define various encodings of the Unicode byte order mark
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(BOM) used in UTF-16 and UTF-32 data streams to indicate the byte order
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used in the stream or file and in UTF-8 as a Unicode signature.
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\constant{BOM_UTF16} is either \constant{BOM_UTF16_BE} or
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\constant{BOM_UTF16_LE} depending on the platform's native byte order,
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\constant{BOM} is an alias for \constant{BOM_UTF16}, \constant{BOM_LE}
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for \constant{BOM_UTF16_LE} and \constant{BOM_BE} for \constant{BOM_UTF16_BE}.
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The others represent the BOM in UTF-8 and UTF-32 encodings.
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\end{datadesc}
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\begin{seealso}
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\seeurl{http://sourceforge.net/projects/python-codecs/}{A
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SourceForge project working on additional support for Asian
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codecs for use with Python. They are in the early stages of
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development at the time of this writing --- look in their
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FTP area for downloadable files.}
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\end{seealso}
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\subsection{Codec Base Classes}
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The \module{codecs} defines a set of base classes which define the
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interface and can also be used to easily write you own codecs for use
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in Python.
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Each codec has to define four interfaces to make it usable as codec in
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Python: stateless encoder, stateless decoder, stream reader and stream
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writer. The stream reader and writers typically reuse the stateless
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encoder/decoder to implement the file protocols.
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The \class{Codec} class defines the interface for stateless
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encoders/decoders.
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To simplify and standardize error handling, the \method{encode()} and
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\method{decode()} methods may implement different error handling
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schemes by providing the \var{errors} string argument. The following
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string values are defined and implemented by all standard Python
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codecs:
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\begin{tableii}{l|l}{code}{Value}{Meaning}
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\lineii{'strict'}{Raise \exception{ValueError} (or a subclass);
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this is the default.}
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\lineii{'ignore'}{Ignore the character and continue with the next.}
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\lineii{'replace'}{Replace with a suitable replacement character;
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Python will use the official U+FFFD REPLACEMENT
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CHARACTER for the built-in Unicode codecs.}
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\end{tableii}
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\subsubsection{Codec Objects \label{codec-objects}}
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The \class{Codec} class defines these methods which also define the
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function interfaces of the stateless encoder and decoder:
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\begin{methoddesc}{encode}{input\optional{, errors}}
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Encodes the object \var{input} and returns a tuple (output object,
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length consumed). While codecs are not restricted to use with Unicode, in
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a Unicode context, encoding converts a Unicode object to a plain string
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using a particular character set encoding (e.g., \code{cp1252} or
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\code{iso-8859-1}).
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\var{errors} defines the error handling to apply. It defaults to
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\code{'strict'} handling.
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The method may not store state in the \class{Codec} instance. Use
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\class{StreamCodec} for codecs which have to keep state in order to
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make encoding/decoding efficient.
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The encoder must be able to handle zero length input and return an
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empty object of the output object type in this situation.
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\end{methoddesc}
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\begin{methoddesc}{decode}{input\optional{, errors}}
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Decodes the object \var{input} and returns a tuple (output object,
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length consumed). In a Unicode context, decoding converts a plain string
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encoded using a particular character set encoding to a Unicode object.
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\var{input} must be an object which provides the \code{bf_getreadbuf}
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buffer slot. Python strings, buffer objects and memory mapped files
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are examples of objects providing this slot.
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\var{errors} defines the error handling to apply. It defaults to
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\code{'strict'} handling.
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The method may not store state in the \class{Codec} instance. Use
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\class{StreamCodec} for codecs which have to keep state in order to
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make encoding/decoding efficient.
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The decoder must be able to handle zero length input and return an
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empty object of the output object type in this situation.
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\end{methoddesc}
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The \class{StreamWriter} and \class{StreamReader} classes provide
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generic working interfaces which can be used to implement new
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encodings submodules very easily. See \module{encodings.utf_8} for an
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example on how this is done.
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\subsubsection{StreamWriter Objects \label{stream-writer-objects}}
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The \class{StreamWriter} class is a subclass of \class{Codec} and
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defines the following methods which every stream writer must define in
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order to be compatible to the Python codec registry.
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\begin{classdesc}{StreamWriter}{stream\optional{, errors}}
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Constructor for a \class{StreamWriter} instance.
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All stream writers must provide this constructor interface. They are
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free to add additional keyword arguments, but only the ones defined
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here are used by the Python codec registry.
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\var{stream} must be a file-like object open for writing (binary)
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data.
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The \class{StreamWriter} may implement different error handling
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schemes by providing the \var{errors} keyword argument. These
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parameters are defined:
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\begin{itemize}
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\item \code{'strict'} Raise \exception{ValueError} (or a subclass);
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this is the default.
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\item \code{'ignore'} Ignore the character and continue with the next.
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\item \code{'replace'} Replace with a suitable replacement character
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\end{itemize}
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\end{classdesc}
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\begin{methoddesc}{write}{object}
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Writes the object's contents encoded to the stream.
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\end{methoddesc}
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\begin{methoddesc}{writelines}{list}
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Writes the concatenated list of strings to the stream (possibly by
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reusing the \method{write()} method).
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\end{methoddesc}
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\begin{methoddesc}{reset}{}
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Flushes and resets the codec buffers used for keeping state.
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Calling this method should ensure that the data on the output is put
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into a clean state, that allows appending of new fresh data without
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having to rescan the whole stream to recover state.
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\end{methoddesc}
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In addition to the above methods, the \class{StreamWriter} must also
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inherit all other methods and attribute from the underlying stream.
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\subsubsection{StreamReader Objects \label{stream-reader-objects}}
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The \class{StreamReader} class is a subclass of \class{Codec} and
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defines the following methods which every stream reader must define in
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order to be compatible to the Python codec registry.
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\begin{classdesc}{StreamReader}{stream\optional{, errors}}
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Constructor for a \class{StreamReader} instance.
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All stream readers must provide this constructor interface. They are
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free to add additional keyword arguments, but only the ones defined
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here are used by the Python codec registry.
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\var{stream} must be a file-like object open for reading (binary)
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data.
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The \class{StreamReader} may implement different error handling
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schemes by providing the \var{errors} keyword argument. These
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parameters are defined:
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\begin{itemize}
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\item \code{'strict'} Raise \exception{ValueError} (or a subclass);
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this is the default.
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\item \code{'ignore'} Ignore the character and continue with the next.
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\item \code{'replace'} Replace with a suitable replacement character.
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\end{itemize}
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\end{classdesc}
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\begin{methoddesc}{read}{\optional{size}}
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Decodes data from the stream and returns the resulting object.
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\var{size} indicates the approximate maximum number of bytes to read
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from the stream for decoding purposes. The decoder can modify this
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setting as appropriate. The default value -1 indicates to read and
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decode as much as possible. \var{size} is intended to prevent having
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to decode huge files in one step.
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The method should use a greedy read strategy meaning that it should
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read as much data as is allowed within the definition of the encoding
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and the given size, e.g. if optional encoding endings or state
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markers are available on the stream, these should be read too.
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\end{methoddesc}
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\begin{methoddesc}{readline}{[size]}
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Read one line from the input stream and return the
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decoded data.
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Unlike the \method{readlines()} method, this method inherits
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the line breaking knowledge from the underlying stream's
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\method{readline()} method -- there is currently no support for line
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breaking using the codec decoder due to lack of line buffering.
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Sublcasses should however, if possible, try to implement this method
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using their own knowledge of line breaking.
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\var{size}, if given, is passed as size argument to the stream's
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\method{readline()} method.
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\end{methoddesc}
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\begin{methoddesc}{readlines}{[sizehint]}
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Read all lines available on the input stream and return them as list
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of lines.
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Line breaks are implemented using the codec's decoder method and are
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included in the list entries.
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\var{sizehint}, if given, is passed as \var{size} argument to the
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stream's \method{read()} method.
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\end{methoddesc}
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\begin{methoddesc}{reset}{}
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Resets the codec buffers used for keeping state.
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Note that no stream repositioning should take place. This method is
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primarily intended to be able to recover from decoding errors.
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\end{methoddesc}
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In addition to the above methods, the \class{StreamReader} must also
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inherit all other methods and attribute from the underlying stream.
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The next two base classes are included for convenience. They are not
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needed by the codec registry, but may provide useful in practice.
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\subsubsection{StreamReaderWriter Objects \label{stream-reader-writer}}
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The \class{StreamReaderWriter} allows wrapping streams which work in
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both read and write modes.
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The design is such that one can use the factory functions returned by
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the \function{lookup()} function to construct the instance.
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\begin{classdesc}{StreamReaderWriter}{stream, Reader, Writer, errors}
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Creates a \class{StreamReaderWriter} instance.
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\var{stream} must be a file-like object.
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\var{Reader} and \var{Writer} must be factory functions or classes
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providing the \class{StreamReader} and \class{StreamWriter} interface
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resp.
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Error handling is done in the same way as defined for the
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stream readers and writers.
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\end{classdesc}
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\class{StreamReaderWriter} instances define the combined interfaces of
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\class{StreamReader} and \class{StreamWriter} classes. They inherit
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all other methods and attribute from the underlying stream.
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\subsubsection{StreamRecoder Objects \label{stream-recoder-objects}}
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The \class{StreamRecoder} provide a frontend - backend view of
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encoding data which is sometimes useful when dealing with different
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encoding environments.
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The design is such that one can use the factory functions returned by
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the \function{lookup()} function to construct the instance.
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\begin{classdesc}{StreamRecoder}{stream, encode, decode,
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Reader, Writer, errors}
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Creates a \class{StreamRecoder} instance which implements a two-way
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conversion: \var{encode} and \var{decode} work on the frontend (the
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input to \method{read()} and output of \method{write()}) while
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\var{Reader} and \var{Writer} work on the backend (reading and
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writing to the stream).
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You can use these objects to do transparent direct recodings from
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e.g.\ Latin-1 to UTF-8 and back.
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\var{stream} must be a file-like object.
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\var{encode}, \var{decode} must adhere to the \class{Codec}
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interface, \var{Reader}, \var{Writer} must be factory functions or
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classes providing objects of the the \class{StreamReader} and
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\class{StreamWriter} interface respectively.
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\var{encode} and \var{decode} are needed for the frontend
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translation, \var{Reader} and \var{Writer} for the backend
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translation. The intermediate format used is determined by the two
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sets of codecs, e.g. the Unicode codecs will use Unicode as
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intermediate encoding.
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Error handling is done in the same way as defined for the
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stream readers and writers.
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\end{classdesc}
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\class{StreamRecoder} instances define the combined interfaces of
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\class{StreamReader} and \class{StreamWriter} classes. They inherit
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all other methods and attribute from the underlying stream.
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