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<H1><A NAME="SECTION006100000000000000000"> </A>
<BR>
4.1 Multiple Interpreters
</H1>
<P>
When working with mod_python, it is important to be aware of a feature
of Python that is normally not used when using the language for
writing scripts to be run from command line. This feature is not
available from within Python itself and can only be accessed through
the <em class='citetitle'><a
href='http://www.python.org/doc/current/api/api.html'
title='C
language API'
>C
language API</a></em>.
<P>
Python C API provides the ability to create <i class="dfn">subinterpreters</i>. A
more detailed description of a subinterpreter is given in the
documentation for the
<em class='citetitle'><a
href='http://www.python.org/doc/current/api/initialization.html'
title='<tt class="cfunction">Py_NewInterpreter()</tt>'
><tt class="cfunction">Py_NewInterpreter()</tt></a></em>
function. For this discussion, it will suffice to say that each
subinterpreter has its own separate namespace, not accessible from
other subinterpreters. Subinterpreters are very useful to make sure
that separate programs running under the same Apache server do not
interfere with one another..
<P>
At server start-up or mod_python initialization time, mod_python
initializes an interpreter called <i class="dfn">main</i> interpreter. The main
interpreter contains a dictionary of subinterpreters. Initially, this
dictionary is empty. With every hit, as needed, subinterpreters are
created, and references to them are stored in this dictionary. The
dictionary is keyed on a string, also known as <i>interpreter
name</i>. This name can be any string. The main interpreter is named
"<tt class="samp">main_interpreter</tt>". The way all other interpreters are named can
be controlled by
<code>PythonInterp*</code> directives. Default behaviour is to name interpreters
using the Apache virtual server name (<code>ServerName</code>
directive). This means that all scripts in the same vrtual server
execute in the same subinterpreter, but scripts in different virtual
servers execute in different subinterpreters with completely separate
namespaces.
<em class='citetitle'><a
href='dir-other-ipd.html'
title='<code>PythonInterpPerDirectory</code>'
><code>PythonInterpPerDirectory</code></a></em> and
<em class='citetitle'><a
href='dir-other-ipdv.html'
title='<code>PythonInterpPerDirective</code>'
><code>PythonInterpPerDirective</code></a></em>
directives alter the naming convention to use the absolute path of the
directory being accessed, or the directory in which the
<code>Python*Handler</code> was encountered, respectively.
<em class='citetitle'><a
href='dir-other-pi.html'
title='<code>PythonInterpreter</code>'
><code>PythonInterpreter</code></a></em> can be used to
force the interpreter name to a specific string overriding any naming
conventions.
<P>
Once created, a subinterpreter will be reused for subsequent requests.
It is never destroyed and exists until the Apache child process dies.
<P>
<div class='seealso'>
<p class='heading'><b>See Also:</b></p>
<dl compact class="seetitle">
<dt><em class="citetitle"><a href="http://www.python.org/doc/current/api/api.html"
>Python C Language API</a></em>
<dd>Python C Language API
</dl>
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<span class="release-info">Release 2.7.8, documentation updated on April 19, 2002.</span>
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