In mypy, what type signature do I write for a function that accepts a type and something of that type as an argument?

Question

I've got this:

def gradient_descent(
  ...
  model_class: Type[Model],
  J: Callable[[np.ndarray, model_class], float],
  ...
):

I want this function to take in a class, and also a function that accepts an instance of that class. However, this gives me the error Name "model_class" is not defined.. I'm assuming that's because mypy doesn't have access to model_class at typechecking time.

Is there any way to achieve this?

@jonrsharpe I don't think so because that answer seems to have nothing to do with mypy. — Eli Rose, Jan 22 '20 at 19:40
That doesn't really matter; you're not getting as far as MyPy, this is a Python problem. — jonrsharpe, Jan 22 '20 at 19:43
@jonrsharpe I know my attempted (wrong) solution results in a Python error, but I still see the problem as a mypy problem. Perhaps I'm missing something, though -- how would I use the techniques described in that answer to solve my problem? — Eli Rose, Jan 22 '20 at 20:43
Ah, it's not quite that, sorry. It's still not a MyPy problem, though, you simply cannot reference other params like that *in Python*. — jonrsharpe, Jan 22 '20 at 20:46
model_class should just be `J: Callable[[np.ndarray, Model], float]` no? — juanpa.arrivillaga, Jan 22 '20 at 20:47
@juanpa.arrivillaga That type would work, but I want to type it more specifically than that. — Eli Rose, Jan 22 '20 at 21:24

jonrsharpe · Accepted Answer · 2020-01-23T14:51:28.017

1

I think what you're looking for is a generic type, e.g.:

T = TypeVar('T', bound=Model) 

def gradient_descent(
  ...
  model_class: Type[T],
  J: Callable[[np.ndarray, T], float],
  ...
):

edited Jan 23 '20 at 14:51

answered Jan 23 '20 at 08:27

jonrsharpe

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Hmm, when I run this I get "'TypeVar' can only have a single constraint." The example gives two constraints, `TypeVar('T', str, bytes)`. It looks like it's functioning as a union type of the two? As opposed to declaring that type T is a subclass of Model. – Eli Rose Jan 23 '20 at 14:48
@EliRose--REINSTATEMONICA ah yes; fixed, see e.g. https://stackoverflow.com/a/50185096/3001761 – jonrsharpe Jan 23 '20 at 14:52

In mypy, what type signature do I write for a function that accepts a type and something of that type as an argument?

1 Answers1