用于getter和setter的Google样式指南属性

问题描述

我很好奇有关属性的Google Python样式指南.

在其中给出了以下示例:

In it, they give the following example:

class Square(object):
    """A square with two properties: a writable area and a read-only perimeter.

    To use:
    >>> sq = Square(3)
    >>> sq.area
    9
    >>> sq.perimeter
    12
    >>> sq.area = 16
    >>> sq.side
    4
    >>> sq.perimeter
    16
    """

    def __init__(self, side):
         self.side = side

    def __get_area(self):
        """Calculates the 'area' property."""
        return self.side ** 2

    def ___get_area(self):
        """Indirect accessor for 'area' property."""
        return self.__get_area()

    def __set_area(self, area):
        """Sets the 'area' property."""
        self.side = math.sqrt(area)

    def ___set_area(self, area):
        """Indirect setter for 'area' property."""
        self.__set_area(area)

    area = property(___get_area, ___set_area,
                    doc="""Gets or sets the area of the square.""")

    @property
    def perimeter(self):
        return self.side * 4

对此我有两个问题:

1. 与直接使用两个下划线相比，使用三个下划线间接" ___get_area和___set_area以及两个下划线有什么好处?

1. What is the benefit of using the three underscore "indirection" ___get_area and ___set_area as well as two underscore ones, over just using the two underscore ones directly?

为什么将property()用作具有这组双重和三次下划线方法的方法，而不是执行以下操作:

Why use property() as a method with this set of double and triple underscore methods, rather than doing something like:

@property
def area(self):
    return self.side ** 2

@area.setter
def area(self, value):
    self.side = math.sqrt(value)

推荐答案

在样式指南中，他们确实给出了原因:

In the style guide they do give a reason:

(其中模板方法DP 是模板方法设计模式(Google的Pythonista非凡专家Alex Martelli的幻灯片).

(where Template Method DP is the Template Method Design Pattern (slides by Alex Martelli, Pythonista extraordinaire at Google).

因此，他们希望为子类提供重写实现的机会，并为property提供三下划线版本来调用double-underscore方法，以便您可以重写这些方法.在这种情况下，您必须拼写错误的名称:

So they want to give subclasses the chance to override the implementation, and give the property the triple-underscore versions to call the double-underscore methods so you can override these. You'd have to spell out the mangled name in that case:

class WonkySquare(Square):
    def _Square__get_area(self):
        return self.square ** 2 + 0.5

显然，提出此方案的人们从来不知道您只能覆盖属性的getter或setter，请参见:

Apparently the people that came up with this scheme never knew that you can override just a getter or setter of a property, see Python overriding getter without setter:

class ProperlySubclassedSquare(Square):
    @Square.area.getter
    def area(self):
        return self.square ** 2 + (0.5 - 0.5)

然后，再次在Python 2.6中添加了getter，setter和deleter装饰器属性.样式指南必须是针对较旧的Python版本编写的.

Then again, the getter, setter and deleter decorator attributes were only added in Python 2.6. The style guide must've been written for an older Python version.

对于2.6及更高版本，请坚持使用@propname.setter模式.

For 2.6 and up, stick to the @propname.setter pattern instead.

这篇关于用于getter和setter的Google样式指南属性的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！