[BUG] not None semantics is not supported in Pure Python mode for object type

[matusvalo]

Describe the bug

Compiling reproducer is run without error. This is inconsistent logic used in Python Objects [1].

[1] https://cython.readthedocs.io/en/latest/src/userguide/extension_types.html#extension-types-and-none

Code to reproduce the behaviour:

import typing
import cython

def bar(i: typing.Optional[object]):
    pass

def bar2(i: object):
    pass

bar(None)
bar([1])
bar2(None)    # This line is executed without error

Expected behaviour

Line bar2(None) should raise following exception:

TypeError: Argument 'i' must not be None

OS

any

Python version

3.9

Cython version

master

Additional context

Cython version:

import cython

def bar(object i):
    pass

def bar2(object i not None):
    pass

bar(None)
bar([1])
bar2(None)   # Here is raised TypeError

Related to #5612

[da-woods]

I don't agree with this (or #5625).

From Cython's point of view, the value of not None is to be able to make optimizations by directly accessing an extension type/built-in type/memoryview without having to check if it's None. There's really no optimizations to be done for object so there's no value in this. The point isn't really to enforce user's type annotations - it's just to make efficient use of them.

I'd also argue that None is an object, so should be accepted for an object/untyped argument.

I'm not sure if non-pure mode accepts object not None. I guess it probably does. But that's probably just an accident more than a useful design.

The documentation looks to be in terms of "extension or Python builtin types". I don't really consider object to be one of those, so I don't think it's inconsistent. But we could clarify it.

[matusvalo]

I don't agree with this (or #5625).

This is fine. I am not 100% sure whether those are real issues (see description). I created the issues because it make sense from consistency stand point - where not None is supported in Cython, I expected that also pure python mode has similar behavior.

Feel free to close the issues without asking.

I'm not sure if non-pure mode accepts object not None.

Cython languages accepts not None in both issues as presented in examples (unless I did a stupid mistake)

[scoder]

Cython languages accepts `not None` in both issues as presented in examples

Then we should warn about them, I think. They make no sense, and users shouldn't put them in their code.

[matusvalo]

Based on the comments I am closing the Issue.