Modernize `test_inspect` by adding real pos-only parameters

[sobolevn]

Right now test_inspect uses several hacks to pretend that some parameters are positional only:

cpython/Lib/test/test_inspect.py

Lines 2465 to 2469 in ecad802

	def test(po, pk, pod=42, pkd=100, *args, ko, **kwargs):
	pass
	sig = inspect.signature(test)
	po = sig.parameters['po'].replace(kind=P.POSITIONAL_ONLY)
	pod = sig.parameters['pod'].replace(kind=P.POSITIONAL_ONLY)

cpython/Lib/test/test_inspect.py

Lines 2519 to 2522 in ecad802

	myparam = MyParameter(name='z', kind=inspect.Parameter.POSITIONAL_ONLY)
	myparams = collections.OrderedDict(sig.parameters, a=myparam)
	mysig = MySignature().replace(parameters=myparams.values(),
	return_annotation=sig.return_annotation)

cpython/Lib/test/test_inspect.py

Lines 3047 to 3052 in ecad802

	def foo(a, b, c, d, **kwargs):
	pass
	sig = inspect.signature(foo)
	params = sig.parameters.copy()
	params['a'] = params['a'].replace(kind=Parameter.POSITIONAL_ONLY)
	params['b'] = params['b'].replace(kind=Parameter.POSITIONAL_ONLY)

cpython/Lib/test/test_inspect.py

Lines 3559 to 3565 in ecad802

	def test(a_po, *, b, **kwargs):
	return a_po, kwargs
	sig = inspect.signature(test)
	new_params = list(sig.parameters.values())
	new_params[0] = new_params[0].replace(kind=P.POSITIONAL_ONLY)
	test.__signature__ = sig.replace(parameters=new_params)

cpython/Lib/test/test_inspect.py

Lines 4160 to 4166 in ecad802

	def test(a_po, b_po, c_po=3, foo=42, *, bar=50, **kwargs):
	return a_po, b_po, c_po, foo, bar, kwargs
	sig = inspect.signature(test)
	new_params = collections.OrderedDict(tuple(sig.parameters.items()))
	for name in ('a_po', 'b_po', 'c_po'):
	new_params[name] = new_params[name].replace(kind=P.POSITIONAL_ONLY)

And maybe others.

It makes code more complex, unclear, and hides the real purpose of these tests.

This is not a design decision, but rather a limitation at the time. Commits are quite old and pos-only syntax was not available 9 and 11 years ago:

• 3f73ca2
• 7c7cbfc

So, I propose to simplify these tests and make them more correct by using explicit pos only parameters.

Plus, we can keep one test like this to be sure that chaning a parameter kind still works as before. But, there's no need in keeping these old tests the way they are.

I will send a PR with the fix 👍

Linked PRs

• gh-103406: Modernize pos-only arguments usage in test_signature #103407
• [3.11] gh-103406: Modernize pos-only arguments usage in test_signature (GH-103407) #103536

[sobolevn]

Since all supported Python versions have pos-only parameters this can be backported to reduce the amount of potential future merge conflicts.

[sobolevn]

Ok, some code samples that I've listed cannot be replaced, because they test different things. But, some - can be.