Repository navigation
Type annotation for map with abstract classes as values #3048
Description
Hello there,
I'm working on a framework with various storage backends. Those backends all implement an abstract base class. The backend classes are stored in a mapping from the backend name to the class implementing that backend.
We want to be able to perform type checking with mypy, and annotate as follows:
import abc
import typing
class A(metaclass=abc.ABCMeta): # The abstract base class
def __init__(self, name: str) -> None:
self.name = name
@abc.abstractmethod
def get_name(self):
pass
class B(A): # Some non-abstract backend
def get_name(self):
return f'B: {self.name}'
class C(A): # Another non-abstract backend
def get_name(self):
return f'C: {self.name}'
backends: typing.Mapping[str, typing.Type[A]] = {
'backend-b': B,
'backend-c': C,
}
if __name__ == '__main__':
backend_cls = backends['backend-c']
# The following line causes an error with mypy:
instance = backend_cls('demo-name')
print(f'Name is: {instance.get_name()}')Running mypy-0.501 gives this error:
typingtest.py:32: error: Cannot instantiate abstract class 'A' with abstract attribute 'get_name'
My question: How can we annotate the mapping backends such that mypy understands it only contains non-abstract subclasses of A?
Activity
gvanrossum commentedon Mar 23, 2017
Should be fixed by #2853, so closing as a duplicate of #1843.