Should Python raise a warning for mutable default arguments?

[Christian Heimes]

MRAB wrote:
> Could there be a new special method __mutable__?

Why should we add a new method?

Seriously, why should Python be cluttered and slowed down to warn about 
mutable arguments. It's neither a design flaw nor a surprising feature 
*ONCE* you have understood how functions work. I agree that the behavior 
may be surprising for a newbie but it's logical once you got the big 
picture.

Christian