TIL you can trick Mypys typechecker using typing.cast

Problem

You might have something like this:

# models.py
class MyModelQuerySet(QuerySet[MyModel]):
    # ...

# views.py
class MyView():
    def get_queryset(self) -> QuerySet:
        # ...
        qs = self.model.stuff()

        # ...
        qs = need_other_stuff.get_other_stuff(queryset=qs)

And when you run Mypy it will spit out the below error:

Incompatible types in assignment (expression has type "_QuerySet[MyModel, MyModel]", variable has type "MyModelQuerySet[MyModel]")  [assignment]

This error points to the following lines:

# ...
qs = self.model.stuff()
# ...
qs = need_other_stuff.get_other_stuff(queryset=qs)

You could explicitly type annotate them but then you'll meet another error

Name "qs" already defined on line x

The solution

Python 3.10 has the cast function, and you wrap the above examples like this:

from typing import cast
# ...
qs = cast(MyModelQuerySet, self.model.stuff())
# ...
qs = cast(MyModelQuerySet, need_other_stuff.get_other_stuff(queryset=qs))

The cast function essentially just takes in a type as the first parameter, your value as the 2nd and simply returns the 2nd one unchanged.

# .../lib/python3.10/typing.py
def cast(typ, val):
    """Cast a value to a type.

    This returns the value unchanged.  To the type checker this
    signals that the return value has the designated type, but at
    runtime we intentionally don't check anything (we want this
    to be as fast as possible).
    """
    return val

A word of caution:

Although doing this is a way to inform mypy your intent it doesn't guarantee that the assigned value will be of the correct type at runtime.

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TIL you can trick Mypys typechecker using typing.cast

Benji 🍙

Problem

The solution

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