Given a class `A` that overloads `__getattr__` ``` class A: def __getattr__(self, key): return 0 ``` An instance of this class is always identified as a dataclass. ``` from dataclasses import is_dataclass a = A() print(is_dataclass(a)) ``` gives the output `True`. Possible fix: check for the instance type. ``` is_dataclass(type(a)) ``` does give the correct answer.

I'm guessing I'm looking up the attribute on the instance, not the class.

I am not sure that it is good idea to accept a type and an instance, but if it is a goal, is_dataclass() should be defined as: def is_dataclass(obj): cls = obj if isinstance(obj, type) else type(obj) return hasattr(cls, _FIELDS) _is_dataclass_instance() should be changed too: def _is_dataclass_instance(obj): return hasattr(type(obj), _FIELDS)

Yeah, I agree it's not an awesome design to work with classes or instances, but it's documented that way. As soon as I write some tests I'll check this in.

 New changeset b0f4dab8735f692bcfedcf0fa9a25e238a554bab by Eric V. Smith in branch 'master': bpo-37868: Improve is_dataclass for instances. (GH-15325) https://github.com/python/cpython/commit/b0f4dab8735f692bcfedcf0fa9a25e238a554bab 

 New changeset 1271ee8187df31debda7c556882a51ec356ca534 by Miss Islington (bot) in branch '3.8': bpo-37868: Improve is_dataclass for instances. (GH-15325) https://github.com/python/cpython/commit/1271ee8187df31debda7c556882a51ec356ca534 

 New changeset 02c1457a036c2af3e91beb952afdb66d9c806435 by Miss Islington (bot) in branch '3.7': bpo-37868: Improve is_dataclass for instances. (GH-15325) https://github.com/python/cpython/commit/02c1457a036c2af3e91beb952afdb66d9c806435