Demonstration of mutability and covariance inference

main.py

from typing import List, Sequence, Dict, Mapping
from typing_extensions import reveal_type

class Animal:
    pass


class Dog(Animal):
    pass

class Cat(Animal):
    pass


def mutate_list(d: List[Animal]):
    pass

def no_mutate_list(d: Sequence[Animal]):
    pass


dog_list = [Dog()]
catdog_list = [Cat(), Dog()]

print(reveal_type(dog_list))    # List[Dog]
print(reveal_type(catdog_list)) # List[Animal]


# error: Argument 1 to "mutate_list" has incompatible type "List[Dog]"; expected "List[Animal]"
# note: "List" is invariant -- see https://mypy.readthedocs.io/en/stable/common_issues.html#variance
# note: Consider using "Sequence" instead, which is covariant
mutate_list(dog_list)

mutate_list(catdog_list)
no_mutate_list(dog_list)



def mutate_dict(d: Dict[str, Animal]):
    pass

def no_mutate_dict(d: Mapping[str, Animal]):
    pass


cat_dict = {"cat": Cat()}                  # Dict[str, Cat]
catdog_dict = {"cat": Cat(), "dog": Dog()} # Dict[str, Animal]

# error: Argument 1 to "mutate_dict" has incompatible type "Dict[str, Cat]"; expected "Dict[str, Animal]"
# note: "Dict" is invariant -- see https://mypy.readthedocs.io/en/stable/common_issues.html#variance
# note: Consider using "Mapping" instead, which is covariant in the value type
mutate_dict(cat_dict)

mutate_dict(catdog_dict)
no_mutate_dict(cat_dict)