用于getter和setter的Google样式指南属性 [英] Google Style Guide properties for getters and setters

问题描述

我很好奇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:

如果不覆盖属性本身，则具有属性的继承可以是非显而易见的.因此，必须确保间接调用访问器方法，以确保属性(使用模板方法DP)调用子类中重写的方法.

Inheritance with properties can be non-obvious if the property itself is not overridden. Thus one must make sure that accessor methods are called indirectly to ensure methods overridden in subclasses are called by the property (using the Template Method DP).

(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IT屋！