Wrong return type inferred for constructors

[marcoeilers]

In Mypy, constructor return types are always None (https://github.com/python/mypy/issues/604), but we seem to infer the type of the class as the return type instead? Or maybe we infer arbitrary types and I just happened to get the type of the class as the return type? Either way, it should be None instead.

[marcoeilers]

Actually, this might be an instance of a larger problem. I think I remember seeing that we infer a return type that is not None for functions that don't actually return anything. I think we shouldn't do that, they should get type None.

[mostafa-abdullah]

The return type of a function with no return is defined as a super type of None. The reason for this was to make this program typeable:

class A:
    def f(self):
        pass

class B(A):
    def f(self):
        return 1

If the method f in class A is restricted to return None, the program will not be typeable due to the variance rules imposed by overriding this method in class B

[caterinaurban]

Can’t we support the @abstractmethod decorator instead?

[mostafa-abdullah]

Done here

Now you can declare a class as abstract by setting its metaclass to ABCMeta (imported from abc module), and declare a method as abstract by decorating it with @abstractmethod. You now get unsat if you try to instantiate an abstract class.

[mostafa-abdullah]

I also modified some unit tests to adapt to this change.

[caterinaurban]

Don't we need to also modify the projects like imp?

[mostafa-abdullah]

I did