Python type hints - better syntax for cast()?

Question

I recently started using type hints in my code, and have so far found them to be (mostly) very helpful.

However, one thing that I really do not like is the syntax to force the type checker to assume that a variable is of a certain type. Given this example:

import itertools
from typing import Iterable, Tuple
x: Iterable[Tuple[str, str]] = itertools.combinations('abc', 2)
# error: Incompatible types in assignment (expression has type "Iterable[Tuple[str, ...]]", variable has type "List[Tuple[str, str]]")

As far as I can tell, the recommended way to work around this is to explicitly cast the object to force the type checker to use the specified type, e.g.:

import itertools
from typing import Iterable, Tuple, cast
x = cast(Iterable[Tuple[str, str]], itertools.combinations('abc', 2))

I personally find this solution to be a bit gross. My primary concern is that, to the inexperienced reader, it is not clear that the cast is purely there to help the static analyzer. (If I didn't already know, I would assume based on the name and context that it is converting and doing a copy into an object of the specified type, when really there is no runtime cost.)

cast looks like any old function call. When I see that a function is being called on a value, I expect the value to be mutated and/or some other side-effects to occur, but in this case the only side effect is that mypy stops complaining. Type hints themselves have a distinct syntax, but I feel that this blurs the lines with a mixture of the new typing syntax and traditional python syntax. (It's already a bit blurry since you have to import the types and can compose them, but that's another discussion.)

Is there an alternative syntax for cast-like behavior? I haven't found anything, but I was hoping for something like:

x1 = itertools.combinations('abc', 2)) # cast: Iterable[Tuple[str, str]] 

x2: Iterable[Tuple[str, str]] = itertools.combinations('abc', 2)) # type: cast

x3: Cast[Iterable[Tuple[str, str]]] = itertools.combinations('abc', 2))

Answer 1

Actually the latest version of Mypy does return the correct type Iterator[Tuple[str, str]].

This change was introduced to Typeshed in PR https://github.com/python/typeshed/pull/4309.

If you cannot update mypy to the latest version you can checkout the latest version from typeshed and use the config option custom_typeshed_dir.

See https://mypy.readthedocs.io/en/stable/config_file.html#confval-custom_typeshed_dir for more details.