On Aug 22, 9:42 am, Steven D'Aprano <[EMAIL PROTECTED] cybersource.com.au> wrote: > 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
I like the idea of calling it to your attention. +1. If a warning, you should be able to silence it with an annotation, decorator, or a module-level flag. Or perhaps, require it be declared explicit, and make it an error. def test(x, a=defmut([])): Python raises an actual error unless default arguments are known immutable or instances of 'defmut'. -- http://mail.python.org/mailman/listinfo/python-list