`isinstance` removes `cls.__dict__["__annotations__"]` in a complex setup with Protocol and ABCMeta

[vfdev-5]

Bug report

Bug description:

import abc
from collections.abc import Iterator
from typing import Protocol

# ---- Expected behavior:
class Steppable(metaclass=abc.ABCMeta):
  pass

class Module:
  @classmethod
  def __init_subclass__(cls, **kwargs) -> None:
    super().__init_subclass__(**kwargs)
    name, annotation, default = "name", str | None, "abc"
    cls.__annotations__[name] = annotation
    setattr(cls, name, default)


class SequenceLayer1(Module, Steppable):
  pass


print(SequenceLayer1.__dict__.get("__annotations__"))
# {'name': str | None}
assert SequenceLayer1.__dict__.get("__annotations__") is not None


# ---- Unexpected behavior:
class CheckpointableIterator(Iterator, Protocol):
  pass

isinstance(CheckpointableIterator, Iterator)

class SequenceLayer2(Module, Steppable):
  pass

print(SequenceLayer2.__dict__.get("__annotations__"))
# None
assert SequenceLayer2.__dict__.get("__annotations__") is not None

• Failing on 3.12 and 3.13
• Passing on 3.11 and 3.14 (For 3.14 it is expected to have __dict__["__annotations__"] missing)

CPython versions tested on:

3.12

Operating systems tested on:

Linux

[picnixz]

I would say it's an issue with either Protocol or ABC cache. Does using the pure Python implementation of abc (_py_abc I think) give the same results? By the way, we won't be changing 3.12 unfortunately but we can possibly fix 3.13.

[typenoob]

I would say it's an issue with either Protocol or ABC cache. Does using the pure Python implementation of abc (_py_abc I think) give the same results? By the way, we won't be changing 3.12 unfortunately but we can possibly fix 3.13.

I can prove _py_abc did not exhibit the same issue.

[typenoob]

cpython/Lib/typing.py

Line 2147 in 60403a5

annotations = getattr(base, '__annotations__', {})

According to this, I simplified the demo. Seems there's side effect in getattr.

import abc

class Module(metaclass= abc.ABCMeta):
  @classmethod
  def __init_subclass__(cls, **kwargs) -> None:
    super().__init_subclass__(**kwargs)
    name, annotation, default = "name", str | None, "abc"
    cls.__annotations__[name] = annotation
    setattr(cls, name, default)


class SequenceLayer1(Module):
  def __init__(self):
    super().__init__()

print(SequenceLayer1.__dict__.get("__annotations__"))
# {'name': str | None}
assert SequenceLayer1.__dict__.get("__annotations__") is not None

getattr(abc.ABCMeta, '__annotations__', {}) # comment this line to suppress the issue

class SequenceLayer2(Module):
  pass

print(SequenceLayer2.__dict__.get("__annotations__"))
# None
assert SequenceLayer2.__dict__.get("__annotations__") is not None

Moreover, in this case _py_abc_ and abc are the same.

[typenoob]

Alright, it likely causes issue on any metaclass.

class MyMeta(type):
  pass

class Module(metaclass = MyMeta):
  @classmethod
  def __init_subclass__(cls) -> None:
    name, annotation = "name", str | None
    cls.__annotations__[name] = annotation


class SequenceLayer1(Module):
  pass

# {'name': str | None}
print(SequenceLayer1.__dict__.get("__annotations__"))

getattr(MyMeta, '__annotations__') # comment this line to suppress the issue

class SequenceLayer2(Module):
  pass

# None
print(SequenceLayer2.__dict__.get("__annotations__"))

[JelleZijlstra]

Note that using __dict__.get to retrieve annotations no longer works reliably on Python 3.14. Use annotationlib.get_annotations instead.

The linked line above

cpython/Lib/typing.py

Line 2147 in 60403a5

annotations = getattr(base, '__annotations__', {})

should be changed to use get_annotations with e.g. the STRING format, because it only cares about the presence of a key in the annotations, not its value.

[JelleZijlstra]

Never mind, I missed that the issue is reported against 3.12/3.13. The code in typing.py was already updated to work better in 3.14.

I think the issue reported here is essentially what's discussed in https://peps.python.org/pep-0749/#pre-existing-bugs . At this point I'm not inclined to change the behavior in 3.13 and lower.

[typenoob]

Do you think we should add the following example to https://peps.python.org/pep-0749/#pre-existing-bugs?
I think it's a little bit different with the existing twos.

class Meta(type): pass

class X(metaclass=Meta):
  @classmethod
  def __init_subclass__(cls) -> None:
    cls.__annotations__

Meta.__annotations__ # important

class Y(X): pass

assert Y.__dict__.get("__annotations__") == {}, Y.__dict__.get("__annotations__") # fails: None