Allow type aliases such X = List[T] if T is a type variable

[JukkaL]

This code should be okay:

from typing import TypeVar, List
T = TypeVar('T')
X = List[T]
def f(x: X) -> None: ...

It should be equivalent to this:

from typing import TypeVar, List
T = TypeVar('T')
def f(x: List[T]) -> None: ...

This was suggested by Guido, "based on the general equivalence in Python of

X = <stuff>
f(X)

to

f(<stuff>)

".

[JukkaL]

This is not included in PEP 484, so closing this.

[gvanrossum]

Whoa, maybe this should be added to PEP 484? I still am in favor of this.

[gvanrossum]

In particular, PEP 484 (https://github.com/ambv/typehinting/blob/master/pep-0484.txt) contains this example:

Vector = Iterable[Tuple[T, T]]

[JukkaL]

What about something like this:

Table = Dict[str, T]

def f(t: Table[int]) -> ...: ...

[gvanrossum]

The implementation in typing.py requires that you write Table[str, int] in the argument attribute. I agree this is a bit arbitrary, but the alternative felt pretty confusing. In an earlier version I was trying to support this to the extreme, allowing e.g.

# T, U, V are TypeVars
A = Tuple[T, U, V]
X = Dict[int, Union[Tuple[U, U], A]]

and then X would have 3 parameters (U, T, V). That felt too complicated, so I retreated on the rule that you can't change the number of parameters at all, and substitutions must be either completely concrete or single type variables, and the resulting type still has the original number of parameters. Maybe we need to find a middle ground, where your example is allowed and does in fact reduce the number of parameters. But you could still have only substitutions that are either completely concrete or a single type variable. An edge case would be X = Dict[T, T] which gives X a single parameter, T.

[berdario]

Just stumbled upon this, the confusing thing is that the error is reported not at the line of the type alias, but at the line at which the TypeVar (T in this case) has been defined:

 error: Invalid type "module.T"

[vitawasalreadytaken]

With mypy 0.4.4 I'm getting the error at the line of the type alias:

from typing import TypeVar, Tuple

T = TypeVar('T')
U = TypeVar('U')

X = Tuple[T, U]

$ mypy test.py
test.py:6: error: Invalid type "test.T"
test.py:6: error: Invalid type "test.U"