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.
Top comments (0)