list of subclasses cannot be assigned to list of superclass

[sarisia]

Describe the bug
I have a Base class and a Child class derived from Base class.
When I try to assign a list of Child instances to List[Base],
the list of Child is recognized as List[Child] and get warning.

To Reproduce
Reproducible code:

from typing import List

class Base():
    pass

class Child(Base):
    pass

class AnotherChild(Base):
    pass


success: List[Base] = [Child(), AnotherChild()]

fail: List[Base] = [Child(), Child()]
fail2: List[Base] = [AnotherChild(), AnotherChild()]

In the code above, I got

Expression of type 'list[Child]' cannot be assigned to declared type 'List[Base]'pyright

for fail and

Expression of type 'list[AnotherChild]' cannot be assigned to declared type 'List[Base]'pyright

for fail2.

Expected behavior
Both Child and AnotherChild is inherited from Base so
List[Child] and List[AnotherChild] should be able to be assigned to List[Base]
without warnings.

Screenshots or Code

VS Code extension or command-line
pyright as a VS Code extension
Pyright 1.0.29

Additional context

[erictraut]

As non-obvious as this might sound, this is actually the correct behavior. That's because "list" and other mutable container types are considered "invariant", so the contained type needs to match exactly. If you use an immutable type like Sequence, your code will work as expected. Immutable container types are considered "covariant", so they accept subclasses.

success1: Sequence[Base] = [Child(), AnotherChild()]
success2: Sequence[Base] = [Child(), Child()]
success3: Sequence[Base] = [AnotherChild(), AnotherChild()]

If you want to understand the theory behind this, check out PEP 483.

You might wonder why the first assignment in your example (the one that assigns the variable called success) works without generating an error. That's because pyright, by default, infers that list expressions that contain mixed types should be treated as List[Any]. If you enable the configuration switch strictListInference in your pyrightconfig.json file -- or optionally add a # pyright: strict comment in your source file, you'll see that an error is generated on this line as well. In strict mode, pyright infers a union type (in the case of your example, List[Union[Child, AnotherChild]]).

[sarisia]

Got it,
sorry for my lack of understanding for python and
thank you for your intelligible explanation