[Python-Dev] PEP 579 and PEP 580: refactoring C functions and methods (original) (raw)

Petr Viktorin encukou at gmail.com
Thu Sep 13 14🔞11 EDT 2018

On 09/13/18 02:22, Jeroen Demeyer wrote:

On 2018-09-13 02:26, Petr Viktorin wrote:

The reference to PEP 573 is premature. It seems to me that PEP 580 helps with the use case of PEP 573. In fact, it implements part of what PEP 573 proposes. So I don't see the problem with the reference to PEP 573. Even if the implementation of PEP 573 changes, the problem statement will remain and that's what I'm referring to.

If you agree I can summarize rationale for "parent", as much as it concerns 580. Sure. I still think that we should refer to PEP 573, but maybe we can summarize it also in PEP 580.

I want to make it clear that PEP 580 doesn't depend on 579. Reviewers don't need to agree with PEP 579 to accept 580.

Here's my proposed rewording: https://github.com/python/peps/pull/775/files?short_path=b34f00e#diff-b34f00eeb75773c32f9b22fd7fee9771

# Using tpprint

The tpprint gimmick is my biggest worry. AFAIK there's no guarantee that a function pointer and Pyssizet are the same size. I'm not actually claiming anywhere that it is the same size.

Indeed, I was thinking ahead here. Backporting this to earlier versions of CPython will not be completely trivial, but let's leave it to Cython.

# Descriptor behavior

I'd say "SHOULD" rather than "MUST" here. The section describes how to implement expected/reasonable behavior, but I see no need to limit that. There is actually an important reason to limit it: it allows code to make assumptions on what get does. This enables optimizations which wouldn't be possible otherwise. If you cannot be sure what get does, then you cannot optimize obj.method(x) to type(obj).method(obj, x)

I see now. Yes, that's reasonable.

"if func supports the C call protocol, then func.set must not be implemented." -- also, delete should not be implemented, right?. Indeed. I write Python but I think C API, so for me these are both really tpdescrset.

PyCCallFASTCALL is not a macro, shouldn't it be named PyCCallFastCall? What's the convention for that anyway? I assumed that capital letters meant a "really know what you are doing" function which could segfault if used badly.

Well, I don't think that's a useful distinction either. This is C; pretty much anything can segfault when used badly.

Macros tend to be "fast": Py_TYPE just gets a member of a struct; Py_INCREF just increments a number. METH_NOARGS is just a number. None of them are very dangerous. IMO, PyTuple_GET_ITEM is not uppercase because it's dangerous, but because it just reaches into memory.

For me, whether something is a function or macro is just an implementation detail (which can change between Python versions) which should not affect the naming.

True. I'm not saying the convention is very strict or useful.

# C API functions

The function PyCFunctionGetFlags is, for better or worse, part of the stable ABI. We shouldn't just give up on it. I'm fine with documenting that it shouldn't be used, but for functions defined using PyCFunctionNew etc. it should continue behaving as before. One solution could be to preserve the "definition time" METH* flags in the 0xFFF bits of ccflags and use the other bits for CCALL*. I'm fine with that if you insist. However, it would be a silly solution to formally satisfy the "stable ABI" requirement without actually helping.

Yes, it's definitely very silly. But that's not a reason to break our promise to the users. After all it's called "stable ABI", not "useful ABI" :)

I agree with your other points that I didn't reply to and will make some edits to PEP 580.

Thank you!

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