类型提示具有耦合继承的列表

Question

I'm using Python 3.7.6 with PyCharm. I want my code to work as a nice internal API with code completion appearing for objects so I want to use typing.

I'm trying to find a good pattern for when:

• There are two groups of classes with each group having it's own inheritance tree
• and objects from one group are composed of lists of objects from other group

(example below)

I've found a way below but it feels like a hack. What's the right (or better) way to do this?

from typing import List, Type

class Leg:
    def step(self):
        print("step")

class DuckLeg(Leg):
    def paddle(self):
        print("splosh")

class Biped:
    def __init__(self, leg_type: Type[Leg]):
        self.legs: List[leg_type] = [leg_type(), leg_type()]

    def walk(self):
        for leg in self.legs:
            leg.step()

class Duck(Biped):
    def __init__(self):
        super().__init__(leg_type=DuckLeg)
        self.legs: List[DuckLeg] = self.legs  # A hack?

my_duck = Duck()
my_duck.walk()              # code-completion appears for .walk()
my_duck.legs[0].paddle()    # code-completion appears for .paddle()

Edit 1: This question is not about where to put the type annotations but how to ensure code-completion works in this context. If the following line is commented-out...

self.legs: List[DuckLeg] = self.legs

...the code will still run because of duck-typing but code-completion will not appear for .paddle() and when manually entered PyCharm code inspection will report: Unresolved attribute reference 'paddle' for class 'Leg'.

Answer 1

Not sure if this actually solves your problem but I think it's cleaner than what you propose:

from typing import List, Type

class Leg:
    def step(self):
        print("step")

class DuckLeg(Leg):
    def paddle(self):
        print("splosh")

class Biped:
    LegType = Leg
    def __init__(self):
        # self.LegType always gives the correct leg type for the instance
        self.legs: List[self.LegType] = [self.LegType(), self.LegType()]

    def walk(self):
        for leg in self.legs:
            leg.step()

class Duck(Biped):
    LegType = DuckLeg
    def __init__(self):
        super().__init__()