官方文档很详细啊！
python官方文档
4.7.1. Default Argument Values¶

The default values are evaluated at the point of function definition in the defining scope, so that

i = 5

def f(arg=i):
    print arg

i = 6
f()

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will print 5.

Important warning: The default value is evaluated only once. This makes a difference when the default is a mutable object such as a list, dictionary, or instances of most classes. For example, the following function accumulates the arguments passed to it on subsequent calls:

def f(a, L=[]):
    L.append(a)
    return L

print f(1)
print f(2)
print f(3)

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This will print

[1]
[1, 2]
[1, 2, 3]

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If you don’t want the default to be shared between subsequent calls, you can write the function like this instead:

def f(a, L=None):
    if L is None:
        L = []
    L.append(a)
    return L

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总结：

1.默认参数在函数初始化的时候就被赋值，因此如果参数默认值为变量，
则默认参数就是函数初始化时的变量所指的对象。
2.函数形参中默认值为容器类对象时，多次调用可能会发生累加效应，
所以形参中一般不建议使用容器类对象做默认值，建议使用None做默认值

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