这两种设置默认值的方法相等吗？

Question

我有以下两种方法来设置对象中参数的默认值： 第一个：

class Test():
    Weight = 12.5

    def __init__(self, weight = Weight):
        self.weight = weight


 Testing = Test()
 print("First testing: {}".format(Testing.weight))

第二个是：

class Test2():
    Weight = 12.5

    def __init__(self, weight = None):
        if weight is None:
            self.weight = Test2.Weight

Testing2 = Test2()
print("Second testing: {}".format(Testing2.weight))

这两个结果的预期格式，他们将打印12.5。 我的问题是，这些方法是否等效，其中一种方法是否比另一种方法更快（我个人的建议是，第一种方法执行起来时间更短）？你喜欢哪一个？还有其他更好的方法吗？ 非常感谢

Answer 1

如果<class>.weight得到更新，则它们不是：

class Test1():
    Weight = 12.5

    def __init__(self, weight = Weight):
        self.weight = weight


testing = Test1()
print("First testing: {}".format(testing.weight))
Test1.Weight = 50
testing2 = Test1()
print("Second testing: {}".format(testing2.weight))

# First testing: 12.5
# Second testing: 12.5


class Test2():
    Weight = 12.5

    def __init__(self, weight = None):
        if weight is None:
            self.weight = Test2.Weight

testing = Test2()
print("First testing: {}".format(testing.weight))
Test2.Weight = 50
testing2 = Test2()
print("Second testing: {}".format(testing2.weight))
# First testing: 12.5
# Second testing: 50

默认参数在方法创建时被求值一次，因此在第一种情况下，默认参数将保持12.5，无论Test1.weight发生什么