How to annotate a user-defined collection in Python?

Question

I've created a collection type:

from collections.abc import Sequence

class MyCollection(Sequence):
    pass

and I want to annotate object of my class like this:

obj: MyCollection[int] = MyCollection()

How to achieve this?

I wrote:

obj: MyCollection[int] = MyCollection()

But IntelliSense only specifies MyCollection not MyCollection[int].

Answer 1

You could use typing it provides runtime support for type hints, for user-defined generic collection, you can use typing.Generic and typing.TypeVar

Here is an example:

from typing import TypeVar, Generic
from collections.abc import Sequence

T = TypeVar('T')

class MyCollection(Sequence, Generic[T]):
    def __init__(self):
        self.data = []

    def __getitem__(self, index):
        return self.data[index]

    def __len__(self):
        return len(self.data)

    def add(self, item: T):
        self.data.append(item)

obj: MyCollection[int] = MyCollection()
obj.add(1)