Understanding usage of TypeVar

Question

When speaking about Generics , python gives the following example:

from collections.abc import Sequence
from typing import TypeVar

T = TypeVar('T')      # Declare type variable

def first(l: Sequence[T]) -> T:   # Generic function
    return l[0]

Could someone please explain what TypeVar does in this case? For example, if it can be anything why not just give it the value Any ? And if it's constrained, why not give it a Union value? In other words, what is the usefulness of using TypeVar(...) ?

I suppose in reviewing the above, it's used when an element of some sort of sub-element? For example, it could be:

[{set1}, {set2}, {set3}, ...]

And that type would be Sequence[set] -> set

But something like this:

[1, 2, 3, ...]

Would have type Sequence[int] -> int . Are there any other usages besides this "item-in-iterable"?

Answer 1

The purpose of the TypeVar in this context is to say that the function returns a specific type that is related to the argument's type.

For example, if you did:

a = first([1, 2, 3]) + "foo"

you would get an error, because in this expression T becomes bound to the type int , and so you'd get an error about adding an int and a str .

If you annotated first with Any types as you describe, this would not produce a mypy error (and hence you'd get a TypeError at runtime instead), because the return value of first would always simply be Any .

See the mypy documentation on generics for a lot more examples of how to use typevars: https://mypy.readthedocs.io/en/stable/generics.html