Cython

An alternative to using the C API directly is to rewrite a C extension using Cython which generates C code using the C API.

Cython allows you to wrap existing C code, but if you want to use Python objects in C or details which are not exposed at the Python level, you still need the Python C API to work with those.

The code that Cython generates is usually faster than the “obvious” C-API functions, so not using the C-API explicitly in Cython has many advantages, such as not relying on CPython version details, simplifying your code, and speeding it up.

Cython is not just a tool to “wrap existing C code”. That’s only one of its major use cases. Speeding up Python code, and writing C code without writing C code, are just as important.

Whether to use Cython or not depends on your use case. It’s not always the obvious choice (otherwise, those other tools would not exist).

XXX write a better rationale why migrating to Cython!

Questions:

• How many popular Python modules use Cython? See Consumers of the Python C API.

• How long would it take to rewrite a C extension with Cython?

• What is the long-term transition plan to reach the “no C API” goal? At least, CPython will continue to use its own C API internally.

• How to deal with backward compatibility?

Small practical issue: Cython is not part of the Python 3.7 standard library yet.

See also cffi and Remove the C API.