Comment by lapinot

Basically afaik the default C API (`Python.h`) is matched to CPython's internal representations, hence it's a pain to support it for alternative implementations and incurs cost penalties. The preferred way to interact with C code in pypy is through cffi (https://cffi.readthedocs.io/en/latest/) and ctypes (which afaik is implemented in pure python on top of cffi in pypy).

Numpy being itself written in C and C++ it is strongly tied to the C API and has a complicated build process. Some stuff works and some don't (didn't try recently). If you're invested in numerical python you should most likely not use pypy but go for stuff like cython (like scipy does).

For psycopg apparently you can use psycopg2cffi (never tried).

> How does pypy handle packages written in other languages, like rust? can I use pypy if I depend on Pydantic?

PyO3 supports pypy so everything should be fine.