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19.2 C Extensions Overview

Because Python itself is coded in C today, compiled Python extensions can be coded in any language that is C-compatible in terms of call stacks and linking. That includes C, but also C++ with appropriate "extern C" declarations (which are automatically provided in Python header files). Python extensions coded in a C-compatible language can take two forms:

  • C modules, which look and feel to their clients like Python module files

  • C types, which behave like standard built-in types (numbers, lists, etc.)

Generally, C modules are used to implement flat function libraries, and C types are used to code objects that generate multiple instances. Because built-in types are really just precoded C extension types, your C extension types can do anything that built-in types can: method calls, addition, indexing, slicing, and so on.[1] In the current Python release, though, types are not quite classes -- you cannot customize types by coding a Python subclass unless you add "wrapper" classes as frontend interfaces to the type. More on this later.

[1] Yes, every time you make an integer or string in Python, you generate a new C type instance object (whether you know it or not). This isn't as inefficient as you may think, though; as we'll see, type operations are dispatched through fast C pointers, and Python internally caches some integers and strings to avoid object creation when possible.

Both C modules and types register their operations with the Python interpreter as C function pointers. In all cases, the C layer is responsible for converting arguments passed from Python to C form, and converting results from C to Python form. Python scripts simply import C extensions and use them as though they were really coded in Python; C code does all the translation work.

C modules and types are also responsible for communicating errors back to Python, detecting errors raised by Python API calls, and managing garbage-collector reference counters on objects retained by the C layer indefinitely -- Python objects held by your C code won't be garbage-collected as long as you make sure their reference counts don't fall to zero. C modules and types may either be linked to Python statically (at build time) or dynamically (when first imported).

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