Goose typing in python

#python #code #datastructure

• It uses collections.abc to create abstract base classes
• When a class inherit some collections like MutableSequence for example, it get all the concrete methods from super class
  • Need to implement the abstract methods to be instantiate
• use of isinstance or issubclass to validate if a class is accepted on a specific part of the code.
• Runtime checks of the code correctness

from abc import ABC, abstractmethod
from math import pi

class Shape(ABC):
    @abstractmethod
    def area(self) -> float: ...

class Circle(Shape):
    def __init__(self, r: float) -> None:
        self.r = r

    def area(self) -> float:
        return pi * self.r ** 2

class Square(Shape):
    def __init__(self, side: float) -> None:
        self.side = side

    def area(self) -> float:
        return self.side ** 2

def paint_needed(shape: Shape, coverage: float) -> float:
    # opcional, mas ilustra o "goose check"
    if not isinstance(shape, Shape):
        raise TypeError("shape não implementa Shape ABC")
    return shape.area() / coverage

sources

• Fluent Python - Luciano Ramalho - 2022 - Second Edition