On 2013-06-21, Chris Angelico <ros...@gmail.com> wrote: > On Sat, Jun 22, 2013 at 4:26 AM, Rick Johnson ><rantingrickjohn...@gmail.com> wrote: >> I could cast a "virtual net" over my poor lemmings before >> they jump off the cliff by throwing an exception: >> >> Traceback (most recent screw-up last): >> Line BLAH in SCRIPT >> def f(x = [None, b, [a, [4]]]): >> ArgumentError: No mutable default arguments allowed! > > So tell me, oh Great and Powerful Wizard of Rick, how is the > interpreter supposed to know which defaults are mutable? I > mean, it's obviously some intrinsic property of the object. > Somehow one thing is clearly immutable, another thing clearly > isn't. Will there be a PyObject_IsImmutable() API? > > Oh! I know. Function argument defaults will now be restricted > to int/float/tuple. That would do it, right? Nobody would be > bothered by little restrictions like that, would they.
I've been around here long enough to have even participated in one of these discussions before. Rick, it's not a wart, It's a gotcha. The reason it's a gotcha is this: In order to predict what will happen correctly, you have to have mastered three separate Python concepts. 1. How name-binding works. 2. How argument passing works, i.e., via name-binding. 3. When default arguments are evaluated. 4. The Python object model. OK, you have to know four things. Curses! I'll come in again. -- Neil Cerutti -- http://mail.python.org/mailman/listinfo/python-list
