On Nov 2, 5:59 am, Steven D'Aprano <st...@remove-this- cybersource.com.au> wrote: > Certainly it's the mediocre programmers who seem to be incapable of > understanding that Python has no way of telling whether arbitrary objects > are mutable or not. > > def foo(x, y=list()): > pass > > Is y a mutabledefaultor not? > > For the benefit of any mediocre programmers out there, be careful before > you answer. This *is* a trick question.
I fail to see your point. You might as well argue that Python has no way of knowing whether it should raise a TypeError in the following example: my_tuple = (1, 2, 3) + list(xrange(4, 7)) Dynamic typing means that these sorts of checks must be delayed until runtime, but that doesn't make them useless or impossible. It seems to me that there is a rather simple case to be made for allowing mutable default arguments: instances of user-defined classes are fundamentally mutable. Disallowing mutable default arguments would mean disallowing instances of user-defined classes entirely. That would be excessive and would violate Python's general rule of staying out of the programmer's way. Cheers, Ian -- http://mail.python.org/mailman/listinfo/python-list
