Sometimes it seems that barely a day goes by without some newbie, or not- so-newbie, getting confused by the behaviour of functions with mutable default arguments. No sooner does one thread finally, and painfully, fade away than another one starts up.
I suggest that Python should raise warnings.RuntimeWarning (or similar?) when a function is defined with a default argument consisting of a list, dict or set. (This is not meant as an exhaustive list of all possible mutable types, but as the most common ones that I expect will trip up newbies.) The warning should refer to the relevant FAQ or section in the docs. What do people think? -- Steven -- http://mail.python.org/mailman/listinfo/python-list
