CODEWITHSUNDEEP
pythonsubclasstype-hintingmypy
Attack68
Attack68

Reputation: 4785

How to structure subclasses with mypy for typing

Say I already have a file with a class that is correctly typed:

#main.py

class Runner(object):
    ...
    def operation(self, arg: str) -> Runner:
        self.attrib = 'something ' + arg 
        return self
    ...

Now I want to separate many of these operation methods sicen my file is becoming too large, so I create a new file subs.py and transfer my code to a mixing:

#main.py

from subs import Mixin
class Runner(Mixin):
    ...

#subs.py

class Mixin(object):
    ...
    def operation(self, arg: str) -> Runner:
        self.attrib = 'something ' + arg 
        return self

The Runner Type is unknowm to subs.py and if I import it then I will be creating circular import dependency, and I dont even think it will work anyway, so Im not sure the correct way to deal with this, in the assumed case that :class:Mixin will only ever be used inherited by the main class.

Upvotes: 0

Views: 196

Answers (1)

chepner
chepner

Reputation: 531708

Once you move the method into the class Mixin, you don't know that operation will return an instance of Runner. Consider:

class Foo(Bar, Mixin):
    ...

f = Foo()
f.operation()  # Returns something that is a Bar and a Mixin, but not a Runner.

The correct type hint is

def operation(self, arg: str) -> Mixin:
    self.attrib = 'something ' + arg
    return self

Upvotes: 1

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