The inspect functions isgeneratorfunction, iscoroutinefunction, isasyncgenfunction can fail on methods that do not have a __code__ attribute:

>>> from types import MethodType
>>> class Callable:
...     def __call__(self, *args):
...         return args
>>> m = MethodType(Callable(), 42)
>>> m()
(42,)
>>> import inspect
>>> inspect.iscoroutinefunction(m)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib64/python3.6/inspect.py", line 186, in iscoroutinefunction
    object.__code__.co_flags & CO_COROUTINE)
AttributeError: 'Callable' object has no attribute '__code__'

This was discovered while working on PEP 575, but it is really an independent issue that should be fixed anyway.