See python/mypy#606. We allow the following, and they are well-defined:

from typing import TypeVar, Dict T = TypeVar('T') U = TypeVar('U') X = Dict[int, str] Y = Dict[T, U] 

But what does this mean?

Z = Dict[T, int] 

In typing.py this currently defines Z as something with two parameters, the second constrained to int or subclasses thereof. But another reasonable interpretation would be that Z has one parameter, corresponding to T only.

The second interpretation is actually more reasonable. (As long as we require that all parameters are either concrete or a simple type variable, not another type expression or alias containing a type variable.)

If we agree, we should define this in the PEP and implement it in typing.py.