TypeVars and the new 3.12 syntax: what happend to co-/contravariances?

Typing
1

(Disclaimer: I actually don’t know what “variance” means. I’m writing this post because I’m curious)

With the new Python 3.12 we have got a new syntax for TypeVars, so instead of:

T = TypeVar("T")
def a_fn(arg: T) -> list[T]: ...

we can write:

def a_fn[T](arg: T) -> list[T]: ...

However with the old way we could also specify variance:

T = TypeVar("T", covariant=False, contravariant=True)

I wonder: why the new syntax doesn’t include variance? Is it because variance can be inferred? If so, how?

2

You are referring to type parameter syntax, which was described in PEP 695.
You will find in the document a “variance inference” section, which answers your question:

This PEP eliminates the need for variance to be specified for type parameters. Instead, type checkers will infer the variance of type parameters based on their usage within a class. Type parameters are inferred to be invariant, covariant, or contravariant depending on how they are used.

1 Like
3

Thanks, it shed a light to me a bit for the topic :slight_smile: