gh-83888: Document that a concrete override need not keep abstract components

[v0ropaev]

Closes #83888, on @gvanrossum's invitation in that thread ("Josh, feel free to submit a PR (make sure it mentions this issue)").

The abc docs let a reader conclude that overriding an abstract read-write property obliges the subclass to supply a setter. It does not. Measured on main:

>>> class C(metaclass=abc.ABCMeta):
...     @property
...     @abc.abstractmethod
...     def foo(self): ...
...     @foo.setter
...     @abc.abstractmethod
...     def foo(self, v): ...
...
>>> C.__abstractmethods__
frozenset({'foo'})
>>> class D(C):
...     @property
...     def foo(self): return 3        # read-only, no setter
...
>>> D.__abstractmethods__
frozenset()
>>> D().foo
3
>>> D().foo = 4
AttributeError: property 'foo' of 'D' object has no setter

So D instantiates happily and the missing setter surfaces much later, as an AttributeError at the assignment.

Why, and why this is a docs change

@rhettinger explained the mechanism and ruled out fixing it:

Right now, ABCMeta only looks at the concrete property. It would have to be modified to scan the next in MRO for an abstract property and then pick apart its component fget, fset, and fdel. That would be a significant jump in complexity with only a minimal payoff.

and @gvanrossum agreed and closed it on that basis:

I agree with Raymond here. This is a job for a static type checker like mypy.

The issue survived as a documentation one, so the text now describes what happens and names that remedy.

Wording

The mechanism is stated as what it actually is, which is not specific to properties: ABCMeta inspects only the object a subclass finally binds to an abstract name, and if that object does not report itself as abstract, the name counts as implemented. Binding foo = 3 clears the abstraction just as effectively as a read-only property does — a property-shaped explanation would have been too narrow.

The abstractproperty block gets a matching sentence, since it is the one place that demonstrated borrowing the remaining components from the base class and so reads as if that were required.

Test

test_concrete_override_drops_abstract_components pins the documented behaviour: the read-only override, its later AttributeError, and the plain-value override. Without it the prose would be the only record of a behaviour nobody has asserted.

It is not tautological — changing the expected D.__abstractmethods__ back to {"foo"} gives 2 failures.

./python.exe -m test test_abc   →  run=76, SUCCESS

No runtime change; Doc/library/abc.rst and Lib/test/test_abc.py only, +45 lines, nothing removed. Since the diff includes a test file the docs label may not pull in skip news on its own — I do not think a Misc/NEWS.d entry is warranted for a docs-and-test change, but say the word and I will add one.

• Issue: Abstract property setter/deleter implementation not enforced, but documented as such #83888

[bedevere-app]

Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.

[read-the-docs-community]

Documentation build overview

📚 cpython-previews | 🛠️ Build #34756980 | 📁 Comparing 68fbeb7 against main (a5b03fa)

  🔍 Preview build  

2 files changed

± library/abc.html
± whatsnew/changelog.html

[v0ropaev]

Closing this myself. @picnixz pointed out on #158152 that I have been opening too many at once and that old issues nobody has revived are the wrong ones to pick up unasked — this is one of those, so it should not be sitting in the queue. The branch stays on my fork if the issue ever comes back to life.

[bedevere-app]

Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.

[v0ropaev]

Reopened. Closing this was my own overreaction to being told I had too many open, not something anyone asked for.