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<H1><A NAME="SECTION006700000000000000000">
4.7 <tt class="module">codecs</tt> --
         Codec registry and base classes</A>
</H1>

<P>


<P>


<P>
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 lookup process.

<P>
It defines the following functions:

<P>
<dl><dt><b><a name='l2h-649'><tt class='function'>register</tt></a></b> (<var>search_function</var>)
<dd>
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 <code>(<var>encoder</var>, <var>decoder</var>, <var>stream_reader</var>,
<var>stream_writer</var>)</code> taking the following arguments:

<P>
<var>encoder</var> and <var>decoder</var>: These must be functions or methods
  which have the same interface as the
  <tt class="method">encode()</tt>/<tt class="method">decode()</tt> methods of Codec instances (see
  Codec Interface). The functions/methods are expected to work in a
  stateless mode.

<P>
<var>stream_reader</var> and <var>stream_writer</var>: These have to be
  factory functions providing the following interface:

<P>
<code>factory(<var>stream</var>, <var>errors</var>='strict')</code>

<P>
The factory functions must return objects providing the interfaces
  defined by the base classes <tt class="class">StreamWriter</tt> and
  <tt class="class">StreamReader</tt>, respectively. Stream codecs can maintain
  state.

<P>
Possible values for errors are <code>'strict'</code> (raise an exception
  in case of an encoding error), <code>'replace'</code> (replace malformed
  data with a suitable replacement marker, such as "<tt class="character">?</tt>") and
  <code>'ignore'</code> (ignore malformed data and continue without further
  notice).

<P>
In case a search function cannot find a given encoding, it should
return <code>None</code>.
</dl>

<P>
<dl><dt><b><a name='l2h-650'><tt class='function'>lookup</tt></a></b> (<var>encoding</var>)
<dd>
Looks up a codec tuple in the Python codec registry and returns the
function tuple as defined above.

<P>
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 <tt class="exception">LookupError</tt> is raised. Otherwise, the codecs
tuple is stored in the cache and returned to the caller.
</dl>

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

<P>
<dl><dt><b><a name='l2h-651'><tt class='function'>open</tt></a></b> (<var>filename, mode</var><big>[</big><var>, encoding</var><big>[</big><var>,
                       errors</var><big>[</big><var>, buffering</var><big>]</big><big>]</big><big>]</big>)
<dd>
Open an encoded file using the given <var>mode</var> and return
a wrapped version providing transparent encoding/decoding.

<P>
<b>Note:</b> 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.

<P>
<var>encoding</var> specifies the encoding which is to be used for the
the file.

<P>
<var>errors</var> may be given to define the error handling. It defaults
to <code>'strict'</code> which causes a <tt class="exception">ValueError</tt> to be raised
in case an encoding error occurs.

<P>
<var>buffering</var> has the same meaning as for the built-in
<tt class="function">open()</tt> function.  It defaults to line buffered.
</dl>

<P>
<dl><dt><b><a name='l2h-652'><tt class='function'>EncodedFile</tt></a></b> (<var>file, input</var><big>[</big><var>,
                              output</var><big>[</big><var>, errors</var><big>]</big><big>]</big>)
<dd>
Return a wrapped version of file which provides transparent
encoding translation.

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

<P>
If <var>output</var> is not given, it defaults to <var>input</var>.

<P>
<var>errors</var> may be given to define the error handling. It defaults to
<code>'strict'</code>, which causes <tt class="exception">ValueError</tt> to be raised in case
an encoding error occurs.
</dl>

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

<P>
<dl><dt><b><a name='l2h-653'><tt>BOM</tt></a></b>
<dd>
<dt><b><a name='l2h-655'><tt>BOM_BE</tt></a></b><dd>
<dt><b><a name='l2h-656'><tt>BOM_LE</tt></a></b><dd>
<dt><b><a name='l2h-657'><tt>BOM32_BE</tt></a></b><dd>
<dt><b><a name='l2h-658'><tt>BOM32_LE</tt></a></b><dd>
<dt><b><a name='l2h-659'><tt>BOM64_BE</tt></a></b><dd>
<dt><b><a name='l2h-660'><tt>BOM64_LE</tt></a></b><dd>
These constants define the byte order marks (BOM) used in data
streams to indicate the byte order used in the stream or file.
<tt class="constant">BOM</tt> is either <tt class="constant">BOM_BE</tt> or <tt class="constant">BOM_LE</tt>
depending on the platform's native byte order, while the others
represent big endian ("<tt class="samp">_BE</tt>" suffix) and little endian
("<tt class="samp">_LE</tt>" suffix) byte order using 32-bit and 64-bit encodings.
</dl>

<P>

<p><hr>
<!--Table of Child-Links-->
<A NAME="CHILD_LINKS"><STRONG>Subsections</STRONG></A>

<UL>
<LI><A NAME="tex2html1866"
  HREF="node80.html" tppabs="http://www.python.org/doc/current/lib/node80.html">4.7.1 Codec Base Classes</A>
<UL>
<LI><A NAME="tex2html1867"
  href="codec-objects.html" tppabs="http://www.python.org/doc/current/lib/codec-objects.html">4.7.1.1 Codec Objects </A>
<LI><A NAME="tex2html1868"
  href="stream-writer-objects.html" tppabs="http://www.python.org/doc/current/lib/stream-writer-objects.html">4.7.1.2 StreamWriter Objects </A>
<LI><A NAME="tex2html1869"
  href="stream-reader-objects.html" tppabs="http://www.python.org/doc/current/lib/stream-reader-objects.html">4.7.1.3 StreamReader Objects </A>
<LI><A NAME="tex2html1870"
  href="stream-reader-writer.html" tppabs="http://www.python.org/doc/current/lib/stream-reader-writer.html">4.7.1.4 StreamReaderWriter Objects </A>
<LI><A NAME="tex2html1871"
  href="stream-recoder-objects.html" tppabs="http://www.python.org/doc/current/lib/stream-recoder-objects.html">4.7.1.5 StreamRecoder Objects </A>
</UL></UL>
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