Consider the following example. The example is contrived but illustrates the point in a runnable example:
class MultiplicatorMixin:
def multiply(self, m: int) -> int:
return self.value * m
class AdditionMixin:
def add(self, b: int) -> int:
return self.value + b
class MyClass(MultiplicatorMixin, AdditionMixin):
def __init__(self, value: int) -> None:
self.value = value
instance = MyClass(10)
print(instance.add(2))
print(instance.multiply(2))
When executed this will give the following output:
12
20
The code works.
But running mypy
on it, yields the following errors:
example.py:4: error: "MultiplicatorMixin" has no attribute "value"
example.py:10: error: "AdditionMixin" has no attribute "value"
I understand why mypy gives this result. But the mixin classes are never used by themselves. They are always used as additional superclasses.
For context, this is a pattern which has been used in an existing application and I am in the process of adding type-hints. And in this case, the errors are false-positives. I am thinking about rewriting the part using the mixins as I don't particularly like it and the same could probably be done with reorganising the class hierarchy.
But I still would like to know how something like this could be properly hinted.
In addition to Campi's answer about the mypy's recommendation of typing mixins with Protocol
:
An alternative to typing the methods' self
s is just inheriting the protocol.
from typing import Protocol
class HasValueProtocol(Protocol):
@property
def value(self) -> int: ...
class MultiplicationMixin(HasValueProtocol):
def multiply(self, m: int) -> int:
return self.value * m
class AdditionMixin(HasValueProtocol):
def add(self, b: int) -> int:
return self.value + b
class MyClass(MultiplicationMixin, AdditionMixin):
def __init__(self, value: int) -> None:
self.value = value
Additionally, if you are TYPE_CHECKING
a Protocol
, and given that you cannot forward reference a parent class (i.e. passing the parent class as a string literal), a workaround would be:
from typing import Protocol, TYPE_CHECKING
if TYPE_CHECKING:
class HasValueProtocol(Protocol):
@property
def value(self) -> int: ...
else:
class HasValueProtocol: ...
class MultiplicationMixin(HasValueProtocol):
def multiply(self, m: int) -> int:
return self.value * m
...