I'm reimplementing `fractions.Fraction` class using C API (https://github.com/lycantropos/cfractions).

And the problem is that I want to use `numbers.Rational` interface in my type checks to add support for user-defined rational numbers.

I see how it's done for `decimal.Decimal` class:
https://github.com/python/cpython/blob/142e5c5445c019542246d93fe2f9e195d3131686/Modules/_decimal/_decimal.c#L2916
but the problem is: I don't see when/where we call `Py_DECREF(Rational)`, so it looks like this class will not be "freed" until the  end of the program.

So my question is: is there any way to define some function which will be called once module is not used?

Hi, you should use the multiphase initialization API which has clear and free functions.
https://www.python.org/dev/peps/pep-0489/#the-proposal

See for example the source code of the Modules/_abc.c extension.

With this setup
https://gist.github.com/lycantropos/f9243dc98e104a13ddd991316e93d31a
I get prompt about module being initialized but that's it: it never gets deleted and reference count for `Rational` keeps increasing. What am I missing?

You are missing m_free.
The clear function is used to break reference counts. In this case, there are no reference counts to be broken, so it is not called.

Your custom_clear function is idempotent, so you can use it in both free and clear and let Python call it either once or twice:
    .m_clear = custom_clear,
    .m_free = custom_clear,

I agree that this is not at all clear from the documentation. I'm going to spend some time organizing the info around the GC API and writing it up.

Thanks for your report, Azat! ✨ 🍰 ✨

I see that I wrote "counts" rather than "cycles" before; that line should be:

   The clear function is used to break reference cycles. In this case, there are no reference cycles to be broken, so it is not called.



Thanks to Pablo for the review & Łukasz for the merges!