Change from ABC to protocol.

[BSchilperoort]

Python nowadays supports "protocols". Protocols are mostly a typing thing, and allow you to specify that a class follows the protocol without explicitly being a subclass.

When using the BMI as an abstract base class, you currently need to use the following syntax:

def update_bmi(bmi: BmiABC) -> None:
    bmi.update()

class BmiImplementation(BmiABC):
    def update(self) -> None:
        ....

update_bmi(BmiImplementation())  # happy type checker

If you don't subclass BMI, type checkers will get angry at this, as BmiImplementation is not a subclass of BmiABC:

def update_bmi(bmi: BmiABC) -> None:
    bmi.update()

class BmiImplementation:
    def update(self) -> None:
        ....

update_bmi(BmiImplementation())  # sad type checker

However, if you specify that Bmi is a subclass of typing.protocol, the following syntax passes type checkers:

def update_bmi(bmi: BmiProtocol) -> None:
    bmi.update()

class BmiImplementation:
    def update(self) -> None:
        ...

update_bmi(BmiImplementation())  # happy type checker

This is because type checkers will check if BmiImplementation follows the BmiProtocol: does it have all methods that are in the protocol, and do all the methods have the correct typing.

To implement this, all that we would need to change is;

from typing import Protocol

class Bmi(Protocol):

Note that you can still subclass from Protocol like you could from an ABC.