Fix "ignored exception in hasattr" in dmypy

[sterliakov]

Fixes #19425. That property has no setter so it should safe to exclude. It can raise in inconsistent state that arises when it is not copied last?

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[JukkaL]

Thanks for the fix! Raising an exception in a property is a bit questionable, so an alternative way to do this would be to make setter a method. But let's merge this since this should fix still the underlying bug.

[ilevkivskyi]

@JukkaL why do we even try to copy properties in replace_object_state()? We have a lot of properties on expressions/statements so it means that there are thousand of AttributeErrors thrown on the setattr() calls because properties are not writeable.

Well there is a TODO item there, maybe we should try addressing it?

[sterliakov]

@ilevkivskyi Compiled and non-compiled versions of properties are vastly different, and we need to support both modes. In particular, properties accessed on class in compiled code aren't instances of builtins.property...

cat <<EOF >c.py
class A:
    @property
    def foo(self) -> int:
        return 0
EOF

$ mypyc c.py
running build_ext
building 'c' extension
creating build/temp.linux-x86_64-cpython-313/build
cc -pthread -fno-strict-overflow -Wsign-compare -Wunreachable-code -DNDEBUG -g -O3 -Wall -fPIC -fPIC -I/home/stas/Documents/Work/mypy/mypyc/lib-rt -I/tmp/tmp.Y6rtvxbGHL/.venv/include -I/home/stas/.local/share/uv/python/cpython-3.13.3-linux-x86_64-gnu/include/python3.13 -c build/__native.c -o build/temp.linux-x86_64-cpython-313/build/__native.o -O3 -g1 -Werror -Wno-unused-function -Wno-unused-label -Wno-unreachable-code -Wno-unused-variable -Wno-unused-command-line-argument -Wno-unknown-warning-option -Wno-unused-but-set-variable -Wno-ignored-optimization-argument -Wno-cpp
creating build/lib.linux-x86_64-cpython-313
cc -pthread -shared -Wl,--exclude-libs,ALL -LModules/_hacl build/temp.linux-x86_64-cpython-313/build/__native.o -L/home/stas/.local/share/uv/python/cpython-3.13.3-linux-x86_64-gnu/lib -o build/lib.linux-x86_64-cpython-313/c.cpython-313-x86_64-linux-gnu.so
copying build/lib.linux-x86_64-cpython-313/c.cpython-313-x86_64-linux-gnu.so -> 
$ python -c 'from c import A; print(isinstance(A.foo, property))'
False
$ rm *.so
$ python -c 'from c import A; print(isinstance(A.foo, property))'
True

Unless mypyc sets some extra attribute on the generated property, I don't know how to detect them without brute force.

[brianschubert]

Hmm, what about inspect.isgetsetdescriptor(A.foo)?

[sterliakov]

@brianschubert They are intentionally included for some reason!

mypy/mypy/util.py

Lines 367 to 409 in f209888

	def get_class_descriptors(cls: type[object]) -> Sequence[str]:
	import inspect # Lazy import for minor startup speed win
	# Maintain a cache of type -> attributes defined by descriptors in the class
	# (that is, attributes from __slots__ and C extension classes)
	if cls not in fields_cache:
	members = inspect.getmembers(
	cls, lambda o: inspect.isgetsetdescriptor(o) or inspect.ismemberdescriptor(o)
	)
	fields_cache[cls] = [x for x, y in members if x != "__weakref__" and x != "__dict__"]
	return fields_cache[cls]
	def replace_object_state(
	new: object, old: object, copy_dict: bool = False, skip_slots: tuple[str, ...] = ()
	) -> None:
	"""Copy state of old node to the new node.
	This handles cases where there is __dict__ and/or attribute descriptors
	(either from slots or because the type is defined in a C extension module).
	Assume that both objects have the same __class__.
	"""
	if hasattr(old, "__dict__"):
	if copy_dict:
	new.__dict__ = dict(old.__dict__)
	else:
	new.__dict__ = old.__dict__
	for attr in get_class_descriptors(old.__class__):
	if attr in skip_slots:
	continue
	try:
	if hasattr(old, attr):
	setattr(new, attr, getattr(old, attr))
	elif hasattr(new, attr):
	delattr(new, attr)
	# There is no way to distinguish getsetdescriptors that allow
	# writes from ones that don't (I think?), so we just ignore
	# AttributeErrors if we need to.
	# TODO: What about getsetdescriptors that act like properties???
	except AttributeError:
	pass