Use multi-phase init

[AA-Turner]

#174.

cc @arigo -- this is a minimal implementation for _cffi_backend.c. Questions:

• Can we remove the PY_VERSION checks? It's hard to test the Python-2 case, and as far as I can tell you only support Python 3.8+.
• Linked to the above, ideally we would consider module isolation, but this requires Python 3-only API that would mean more #if branching, so I've avoided for now.
• There's a use of single-phase init (PyModule_Create) in vengine_cpy.py, but the file is marked as deprecated. Is the file still used / should I update this?
• _my_Py_InitModule() and b_init_cffi_1_0_external_module() will be tricky to migrate. Would it be possible to use the dynamic creation functions here instead?

A

[arigo]

cc @nitzmahone

[mattip]

@ngoldbaum, is this part of #178 or are these PRs orthagonal?

[mattip]

Linked to the above, ideally we would consider module isolation,

I think that is fine. Python changed the documentation: it used to state that mult-phase init implied module isolation, now it states that module isolation is recommended but not required.

[mattip]

There's a use of single-phase init (PyModule_Create) in vengine_cpy.py, but the file is marked as deprecated. Is the file still used / should I update this?

I see the engine choice logic is here. It seems vengine_cpy.VCPythonEngine is used if

• We are not on PyPy
• ffi._backend is not _cffi_backend. As far as I can tell this is only used in tests to try out the ctypes backend, and should not happen in "real life".

@arigo is that a correct assesment? If true, then I would not worry about vengine_cpy too much.

[mattip]

Can we remove the PY_VERSION checks? It's hard to test the Python-2 case, and as far as I can tell you only support Python 3.8+.

I think that is fine to drop Python-2 in the c-based backend code. We keep some python2-compatible code in other places to attempt to still work with PyPy-2, but certainly the c-based backend here is only used on CPython. It might be nice to do this in a separate PR.

In any case, there should be a short blurb about these changes in doc/source/whatsnew.rst since they are user-facing.

[mattip]

Typo

Suggested change

	.m_methds = FFIBackendMethods,
	.m_methods = FFIBackendMethods,

[arigo]

* We are not on PyPy

* `ffi._backend` is not `_cffi_backend`. As far as I can tell this is only used in tests to try out the ctypes backend, and should not happen in "real life".

@arigo is that a correct assesment? If true, then I would not worry about vengine_cpy too much.

No, vengine_cpy is used if we are not on PyPy, and if ffi._backend is _cffi_backend (which is usually true).

[ngoldbaum]

@ngoldbaum, is this part of #178 or are these PRs orthagonal?

They're orthogonal.