Python type hinting to return class in function

python-3.x, type-hinting

Solution

The annotation is meaningless, because your function returns instances of a new, distinct `Bar` class each time it is called.

In other words, there is no one `Bar` class against which you can check the type. The result of `type(foo(1)) is type(foo(2))` is always going to be false, so a type check against `type(foo(1))` is always going to fail when you pass in `foo(2)` too.

If you want type checking to apply here, you'll need to declare a base class:

class BaseBar: pass

def foo(i: int) -> BaseBar:
    class Bar(BaseBar):
        def __init__(i: int):
            self.i = i
    return Bar(i)

To make this a bit more readable, the base class could just be named `Bar` and the function could produce a `BarImpl` or `ConcreteBar` or similar. That way you can keep using `Bar` in type annotations everywhere to reduce confusion.

You don't actually seem to need to create a new class each time; nothing in the class is parameterised or uses closures; if this is true for your actual code, then you could just define it outside the function:

class Bar:
    def __init__(i: int):
        self.i = i

def foo(i: int) -> Bar:
    return Bar(i)

Problem

Original source