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 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 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 virtual 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 process dies.

You can find out the name of the interpreter under which you're running by peeking at req.interpreter.

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 "main_interpreter". 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 "main_interpreter", a range of Python errors can arise, each typically referring to code being run in restricted mode.