TypeVar in python

Question

I ran the following code in Jupyter Notebook:

%load_ext nb_mypy
from typing import Any, List, Union, TypeVar
T = TypeVar("T",int,str)

def first(container: List[T]) -> T:
    return container[2]

ls: List[Any] = [1,"hello",("hello",)]
first(ls)

And the result was:

('hello',)

Here, I constrained the variable T to represent only str or int types. Then, I constrained the container parameter of the function. I believed that the elements in container could only be int or str, but when I tried to pass a list ls containing tuples, mypy did not report an error. I cannot understand why.

Doesn't List[T] mean that container can only contain variables of type T? Here, I also constrained the return value of the function to be of type T, but it seems to have had no effect, as the function still returned a tuple, and mypy did not report any errors.

Answer 1

You've annotated ls as List[Any]. Any is Mypy's escape hatch, that tells it not to type check, and to accept any operation on the object. For instance,

x: Any = 3
x + "foo"

type checks just fine. If you remove the List[Any] hint, or change it list[object] or list[int | str | tuple[str]] (Python 3.10+ syntax), you get an error as you'd expect.

The answer to this question may be of help.