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). Regardless of the implementation language, the compiled Python extensions language can take two forms:

Generally, C extension modules are used to implement flat function libraries, and they wind up appearing as importable modules to Python code (hence their name). C extension types are used to code objects that generate multiple instances and may optionally support expression operators much like classes. 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.^[*]

Moreover, C extension types today may provide a class-like interface, and so can support customization by either subclassing or coding “wrapper” classes as frontend interfaces to the type. For instance, as we saw in Chapter 20, the Python list object may now be customized by direct subclassing.

To make the interface work, both C modules and types must provide a layer of “glue” code that translates calls and data between the two languages. This layer registers C-coded 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 for converting results from C to Python form. Python scripts simply import C extensions and use them as though they were really coded in Python. Because C code does all the translation work, the interface is very seamless and simple in Python scripts.

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. Once coded, C modules and types may be linked to Python either statically (by rebuilding Python) or dynamically (when first imported). Thereafter, the C extension becomes another toolkit available for use in Python scripts.