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.

There is no typing contrait in Python. Even if you declare something as some type, you can assign later whatever you want. Same for function/method parameters. — Itération 122442, May 06 '23 at 11:12

score 1 · Accepted Answer · answered May 06 '23 at 12:23

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.

TypeVar in python

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