Type hints for classes implementing an interface

Question

Suppose I have an interface A with a single function.

class A(metaclass=ABCMeta):

    @abstractmethod
    def spam(self, x: int) -> str:
        pass

There are classes B and C that implement this interface, but they will not be directly initialized. I will have some factory method (say) that will return to me a suitable object implementing A. So in this case, when I implement spam in B and C, should I repeat the type hints? Practically, since B and C aren't directly used, the type hints for A seem sufficient. But I'm curious about the best practice in this situation; and if there are other issues to be considered.

if you're performing static analysis, the safest option is to supply hints everywhere. This question really depends on how static analyzers are implemented, I don't believe that any analyzers will realize that your class implements an abstractmethod and, as a result, use the types supplied there. — Dimitris Fasarakis Hilliard
– Dimitris Fasarakis Hilliard, Commented Mar 22, 2017 at 21:54
See stackoverflow.com/questions/54087885/… and stackoverflow.com/questions/25183424/… — Wesley Smith
– Wesley Smith, Commented Aug 20, 2019 at 15:01

max · Accepted Answer · 2017-04-17 05:42:00Z

1

Presumably, since you implement B.spam, it won't be a trivial implementation (otherwise, why bother overriding A.spam?). So you probably should type check the body of B.spam. In that case, you need to provide type hints for the arguments and return values of B.spam; otherwise, mypy will treat those types as Any.

Note that if you do provide type hints, mypy will check that the type of B.spam is compatible with A.spam using its rules for subclassing, but only if B derives from A non-virtually; if you just A.register(B), mypy will ignore the inheritance relationship entirely. This is consistent with how the runtime will not look up .spam in the virtual base classes.

answered Apr 17, 2017 at 5:42

max

52.7k60 gold badges224 silver badges379 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

max Over a year ago

Any clarification as to why my answer is wrong would be welcome.

Collectives™ on Stack Overflow

Type hints for classes implementing an interface

1 Answer 1

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related