What type should I set instead of TYPE1 and TYPE2 for x, y values?

x: TYPE1 = list
y: TYPE2 = list[int]

For TYPE1 case, if I set

type T = int | str | bool
x: type[list[T]] = list

mypy returns no errors, but why everything is ok here if value - "list" has no type specified?

And for TYPE2 I have no idea what it should be set to

Ok, so I end up with

type TT = int | str | bool | float
type GenericIterableType[T: TT] = list[T] | tuple[T, ...] | set[T]
t: type[GenericIterableType[int] | GenericIterableType[str]] = list[str]

But is it possible to not repeat GenericIterableType for every type in TT ?

What type should I set instead of TYPE1 and TYPE2 for x, y values?

x: TYPE1 = list
y: TYPE2 = list[int]

For TYPE1 case, if I set

type T = int | str | bool
x: type[list[T]] = list

mypy returns no errors, but why everything is ok here if value - "list" has no type specified?

And for TYPE2 I have no idea what it should be set to

Ok, so I end up with

type TT = int | str | bool | float
type GenericIterableType[T: TT] = list[T] | tuple[T, ...] | set[T]
t: type[GenericIterableType[int] | GenericIterableType[str]] = list[str]

But is it possible to not repeat GenericIterableType for every type in TT ?

• python
• python-typing

• Python is a dynamic language and their authors don't care about types. Usually, there are much less errors as compared to c++, java, etc. Also see here. – aaa Commented Jan 29 at 21:04
• Variable types is for IDE code validation/autocompletion purposes only. Runtime-wise it is just old regular list. – Organis Commented Jan 29 at 21:10
• 2 @aaa mypy absolutely cares about types – juanpa.arrivillaga Commented Jan 29 at 22:12
• 1 @Organis they mention mypy explicitly, so they are using static type checking – juanpa.arrivillaga Commented Jan 29 at 22:12
• GenericIterableType ... if that's what you want, you should just use collections.abc.Iterable – juanpa.arrivillaga Commented Jan 30 at 16:16

1 Answer 1

mypy returns no errors, but why everything is ok here if value - "list" has no type specified?

Because the value is fine? Without reified generic, the value list works for any type[list[X]] you want. You can see that the type-checking works if you use x: https://mypy-play/?mypy=latest&python=3.12&flags=no-warn-no-return%2Cwarn-redundant-casts%2Cwarn-return-any%2Cwarn-unreachable&gist=86ac2f94aff7b3f2f923162b9c9f8988

type T = int | str | bool
x: type[list[T]] = list

l1 = x()
l1.append(1)
l1.append("xxx")

l2 = x()
l2.append(None)

mypy triggers an error on the last line, because None is not a valid value for a list[T].

Incidentally int | bool is redundant, for historical reasons in Python bool is a subtype of int, so int | bool = int.

And for TYPE2 I have no idea what it should be set to

type[list[int]], what else would it be?