Python 默认参数处理方式

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.

@Roberto Liffredo

Least Astonishment in Python: The Mutable Default Argument

我真觉得这是一个特别 SB 的设定。