python - 避免对子类进行重复验证

Question

我有以下 Python 3.6 代码:

import abc
import math
from abc import ABC


class Shape(ABC):
    @abc.abstractmethod
    def draw(self, area):
        pass


class Square(Shape):
    def draw(self, area):
        if area < 0:
            raise ValueError('Area must be greater or equal than 0.')

        print('Square. Side length: %s' % math.sqrt(area))


class Circle(Shape):
    def draw(self, area):
        if area < 0:
            raise ValueError('Area must be greater or equal than 0.')

        print('Circle. radius: %s' % math.sqrt(area / math.pi))

如何避免对每个子类重复相同的验证？

Answer 1

使用基类创建一个在子类中使用的检查方法

import abc
import math
from abc import ABC


class Shape(ABC):
    @abc.abstractmethod
    def draw(self, area):
        pass
    def _check_area(area):
        if area < 0:
            raise ValueError('Area must be greater or equal than 0.')


class Square(Shape):
    def draw(self, area):
        self._check_area(area)
        print('Square. Side length: %s' % math.sqrt(area))


class Circle(Shape):
    def draw(self, area):
        self._check_area(area)
        print('Circle. radius: %s' % math.sqrt(area / math.pi))

(我使用单个下划线前缀向潜在用户表明这是一个内部方法，并不是类接口(interface)的真正组成部分)

或者甚至更好:创建一个 _draw 内部方法，该方法将是抽象的，而不是 draw 方法。因此，您可以在 draw 中检查区域并在基类中调用 _draw:

import abc
import math
from abc import ABC


class Shape(ABC):
    def draw(self, area):
        if area < 0:
            raise ValueError('Area must be greater or equal than 0.')
        self._draw

    @abc.abstractmethod
    def _draw(self, area):
        pass


class Square(Shape):
    def _draw(self, area):
        print('Square. Side length: %s' % math.sqrt(area))


class Circle(Shape):
    def _draw(self, area):
        print('Circle. radius: %s' % math.sqrt(area / math.pi))