List[Union[...]] is not always satisfied by valid inputs.

[tomchristie]

When a list of unions is used, mypy will incorrectly error when a subset of the possible union types is used...

example.py:

from typing import List, Union

def mean(numbers: List[Union[int, float]]) -> float:
    return sum(numbers) / len(numbers)

some_numbers = [1, 2, 3, 4]
mean(some_numbers)

console:

$ mypy --version
mypy 0.470
$ mypy example.py 
example.py:7: error: Argument 1 to "mean" has incompatible type List[int]; expected List[Union[int, float]]

Presumably the inferred List[int] ought to be able to satisfy List[Union[int, float]], right?

[ilevkivskyi]

Presumably the inferred List[int] ought to be able to satisfy List[Union[int, float]], right?

No, List is invariant, and therefore List[int] is not a subtype (not compatible with) List[Union[int, float]] (the latter actually is just List[float], see PEP 484). The reason is that lists are mutable and therefore it is not safe to pass something of type List[int], where List[float] (or List[Union[int, float]]) is expected.

See the docs for such situations. In your case I would recommend annotating your function as:

def mean(numbers: Sequence[float]) -> float:

(since you don't mutate numbers.)

[tomchristie]

Thanks for the clarification. 😄
(& apologies for the noise!)

[gvanrossum]

This should also be called out much earlier in the mypy docs. It's a very common "bug" report.

[ilevkivskyi]

I think what Jukka proposed in #3352 (including link to docs in error messages) will solve the problem.

[ierezell]

Sorry for the necroBump but I'm not able to have a list from Literals (as the exemple here) and I guess it's the same topic.

I Feel that something is possible but I cannot make the typing system satisfied with that

For exemple :

MyListType = Literal['dogs', 'cats', 'crocodiles', 'ducks', 'bird', 'pony', 'fish'] # Tried with Union[Literal,Literal...] as well
my_list : List[MyListType] = ['cats', 'crocodiles', 'ducks'] # Tried with Sequence as well and putting all elements in the MyListType

I guess it's comming from Type "str" cannot be assigned to type "MyListType" which should...

[gvanrossum]

@ierezell I don't get an error for that input. Please see Gitter for more help.

[Godsmith]

Presumably the inferred List[int] ought to be able to satisfy List[Union[int, float]], right?

No, List is invariant, and therefore List[int] is not a subtype (not compatible with) List[Union[int, float]] (the latter actually is just List[float], see PEP 484). The reason is that lists are mutable and therefore it is not safe to pass something of type List[int], where List[float] (or List[Union[int, float]]) is expected.

To explain why this is unsafe, to help people like me who have never heard the term invariant before, say that you have the following code:

def add_float(numbers: List[Union[int, float]]):
    numbers.append(4.5)

ints = [1, 2, 3]  # type: List[int]

If you then do

add_float(ints)

you will end up with ints containing [1, 2, 3, 4.5] even though in the type definition it says that ints should only contain integers! And that is what they meant when they said passing the list into the method was "unsafe".