add by zhj: 在Python文档中清楚的说明了默认参数是怎么工作的，如下
"Default parameter values are evaluated when the function definition is executed. This means that the expression is evaluated once, when the function is defined, and that the same “pre-computed” value is used for each call. This is especially important to understand when a default parameter is a mutable object, such as a list or a dictionary: if the function modifies the object (e.g. by appending an item to a list), the default value is in effect modified. This is generally not what was intended. A way around this is to use None as the default, and explicitly test for it in the body of the function
def whats_on_the_telly(penguin=None):
if penguin is None:
penguin = []
penguin.append("property of the zoo")
return penguin
"
参见https://docs.python.org/2/reference/compound_stmts.html#function-definitions
这个问题的答案在 StackOverflow 上可以找到答案。这里将得票数最多的答案最重要的部分摘录如下：
在这个回答中，答题者认为出于Python编译器的实现方式考虑，函数是一个内部一级对象。而参数默认值是这个对象的属性。在其他任何语言中，对象属性都是在对象创建时做绑定的。因此，函数参数默认值在编译时绑定也就不足为奇了。
然而，也有其他很多一些回答者不买账，认为即使是first-class object也可以使用closure的方式在执行时绑定。
甚至还有反驳者抛开实现逻辑，单纯从设计角度认为：只要是违背程序猿基本思考逻辑的行为，都是设计缺陷！下面是他们的一些论调：
> Sorry, but anything considered “The biggest WTF in Python” is most definitely a design flaw. This is a source of bugs for everyone at some point, because no one expects that behavior at first - which means it should not have been designed that way to begin with.
好吧，这么看来，如果没有来自于Python作者的亲自澄清，这个问题的答案就一直会是一个谜了。