4.1 Multiple Interpreters

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 C language API.

Python C API provides the ability to create subinterpreters. A more detailed description of a subinterpreter is given in the documentation for the Py_NewInterpreter() 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..

At server start-up or mod_python initialization time, mod_python initializes an interpreter called main 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 interpreter name. This name can be any string. The main interpreter is named "main_interpreter". The way all other interpreters are named can be controlled by PythonInterp* directives. Default behaviour is to name interpreters using the Apache virtual server name (ServerName 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. PythonInterpPerDirectory and PythonInterpPerDirective directives alter the naming convention to use the absolute path of the directory being accessed, or the directory in which the Python*Handler was encountered, respectively. PythonInterpreter can be used to force the interpreter name to a specific string overriding any naming conventions.

Once created, a subinterpreter will be reused for subsequent requests. It is never destroyed and exists until the Apache child process dies.