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：

Actually, this is not a design flaw, and it is not because of internals, or performance.
It comes simply from the fact that functions in Python are first-class objects, and not only a piece of code.
As soon as you get to think into this way, then it completely makes sense: a function is an object being evaluated on its definition; default parameters are kind of “member data” and therefore their state may change from one call to the other - exactly as in any other object.
In any case, Effbot has a very nice explanation of the reasons for this behavior in Default Parameter Values in Python.
I found it very clear, and I really suggest reading it for a better knowledge of how function objects work.

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。

This is not a design flaw. It is a design decision; perhaps a bad one, but not an accident. The state thing is just like any other closure: a closure is not a function, and a function with mutable default argument is not a function.

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.

The phrases “this is not generally what was intended” and “a way around this is” smell like they’re documenting a design flaw.

好吧，这么看来，如果没有来自于Python作者的亲自陈清，这个问题的答案就一直会是一个谜了。