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'Any' with a bound? #8981
Description
Activity
https://docs.python.org/3/library/typing.html#typing.TypeVar
Alternatively, a type variable may specify an upper bound using bound=. This means that an actual type substituted (explicitly or implicitly) for the type variable must be a subclass of the boundary type [...]
I think it should report an error for
c: C[Any] = C()rather thanc.x = 'abc', sinceAnyis a superset of int, not a subset.https://docs.python.org/3/library/typing.html#typing.TypeVar
Alternatively, a type variable may specify an upper bound using bound=. This means that an actual type substituted (explicitly or implicitly) for the type variable must be a subclass of the boundary type [...]
I think it should report an error for
c: C[Any] = C()rather thanc.x = 'abc', sinceAnyis a superset of int, not a subset.The
typingmodule documentation you linked describesAnyas:Special type indicating an unconstrained type.
- Every type is compatible with
Any. Anyis compatible with every type.
Since it's compatible in both directions, I think
C[Any]is valid.- Every type is compatible with
Plain
Cis same asC[Any]. Should that beC[bound of T]instead?This issue appears to be based on a misunderstanding of how
Anyworks in the Python type system. For details about howAnyis meant to work and why it exists, refer to PEP 483.The use of
Anyas a type argument is especially important in cases where the type parameter is invariant, as the use of any concrete type such asobjectorbound of Twill result in invariant type incompatibilities.Mypy is doing the right thing in how it's treating
Any, and it would be inappropriate to emit an error in any of the above cases.I recommend closing this issue.
When mypy is run on the following code:
It will not report any errors. However, since
Thasintas a bound, assigning a string toxshould be considered invalid even if we don't know the exact type ofx.It seems that when
Anyis substituted for the type argument ofC, the knowledge of the bound is lost. PerhapsAnyTypecould be given a bound,objectby default, to restrict which typesAnycan represent in a particular context.