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<H1><A NAME="SECTION006100000000000000000"></A><A NAME="pyapi-interps"></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="Py_NewInterpreter()"
 ><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 request, as needed, subinterpreters
are created, and references to them are stored in this dictionary. The
dictionary is keyed on a string, also known as <em>interpreter
name</em>. 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
virtual 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="PythonInterpPerDirectory"
 ><code>PythonInterpPerDirectory</code></a></em> and
<em class="citetitle"><a
 href="dir-other-ipdv.html"
 title="PythonInterpPerDirective"
 ><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="PythonInterpreter"
 ><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 process dies.

<P>
You can find out the name of the interpreter under which you're
running by peeking at <tt class="member">req.interpreter</tt>.

<P>
Note that if any third party module is being used which has a C code
component that uses the simplified API for access to the Global
Interpreter Lock (GIL) for Python extension modules, then the interpreter
name must be forcibly set to be "<tt class="samp">main_interpreter</tt>". This is necessary
as such a module will only work correctly if run within the context of
the first Python interpreter created by the process. If not forced to
run under the "<tt class="samp">main_interpreter</tt>", a range of Python errors can arise,
each typically referring to code being run in <em>restricted mode</em>.

<P>
<div class="seealso">
  <p class="heading">See Also:</p>

  <dl compact="compact" class="seetitle">
    <dt><em class="citetitle"><a href="http://www.python.org/doc/current/api/api.html"
        >Python C Language API</a></em></dt>
    <dd>Python C Language API</dd>
  </dl>
  <dl compact="compact" class="seetitle">
    <dt><em class="citetitle"><a href="http://www.python.org/peps/pep-0311.html"
        >PEP 0311 - Simplified Global Interpreter Lock Acquisition for Extensions</a></em></dt>
    <dd>PEP 0311 - Simplified Global Interpreter Lock Acquisition for Extensions</dd>
  </dl>
</div>

<P>

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<span class="release-info">Release 3.3.1, documentation updated on January 29, 2007.</span>
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