Annotating a UserList subclass

[JakobDev]

I'm using collections.UserList to create a custom list class. See this example.

import collections


class TestClass(collections.UserList):
    def __init__(self) -> None:
        super().__init__()

    def lower_all(self) -> None:
        for count, s in enumerate(self.data):
            self.data[count] = s.lower()


test = TestClass()

test.append("Hello")
test.append("World")

test.lower_all()

print(test[0])

In this example, TestClass should always be a list of strings. How can I annotate that?

[Daverball]

It depends on the Python version, if you're still on 3.8 you will need to add some if TYPE_CHECKING blocks to deal with the fact that ABCMeta (or rather collections.abc.MutableSequence, which collections.UserList inherits from) is not subscriptable.

Starting with 3.9 it should be as simple as:

import collections

class TestClass(collections.UserList[str]):
    def __init__(self) -> None:
        super().__init__()

    def lower_all(self) -> None:
        for count, s in enumerate(self.data):
            self.data[count] = s.lower()

With 3.8 you would need to do something like the following:

import collections
from typing import TYPE_CHECKING
if TYPE_CHECKING:
    BaseList = collections.UserList[str]
else:
    BaseList = collections.UserList

class TestClass(BaseList):
    def __init__(self) -> None:
        super().__init__()

    def lower_all(self) -> None:
        for count, s in enumerate(self.data):
            self.data[count] = s.lower()

[JakobDev]

Thanks for the answer. It looks like, this is not supported by mypy. If you check this Code with mypy:

import collections


class TestClass(collections.UserList):
    def __init__(self) -> None:
        super().__init__()

    def lower_all(self) -> None:
        for count, s in enumerate(self.data):
            self.data[count] = s.lower()


test = TestClass()

test.append("Hello")
test.append("World")

test.lower_all()

print(test[0])

test2: list[str] = []

test[0].foo()
test2[0].foo()

YOu get only the error for the test2 but not for test.

[Daverball]

collections.UserList just like list is generic. If you omit the type it will default to Any, so once a type is Any anything will work.

You can see that by using reveal_type(test[0]) which will be Any. This is also why you are not getting any errors inside the lower_all method, a reveal_type(self.data[count]) will also give you Any.

So if you want to have the same behavior as with a list[str] you will need to specify collections.UserList[str] just as in my example code above.

If you want mypy to warn you about unbound generics you can use the --disallow-any-generics option. Then you will get an error at the line class TestClass(collections.UserList): for not specifying the type of the UserList.

[JakobDev]

Sorry, this was absolutely stupid from me 🤦 . I just forget to add it while testing. Now it works.