On Tue, Jul 7, 2009 at 1:04 PM, kj<no.em...@please.post> wrote: > > > I'm having a hard time coming up with a reasonable way to explain > certain things to programming novices. > > Consider the following interaction sequence: > >>>> def eggs(some_int, some_list, some_tuple): > ... some_int += 2 > ... some_list += [2] > ... some_tuple += (2,) > ... >>>> x = 42 >>>> y = (42,) >>>> z = [42] >>>> eggs(x, y, z) >>>> x > 42 >>>> y > (42,) >>>> z > [42, 2] >>>> > > How do I explain to rank beginners (no programming experience at > all) why x and y remain unchanged above, but not z? > > Or consider this one: > >>>> ham = [1, 2, 3, 4] >>>> spam = (ham,) >>>> spam > ([1, 2, 3, 4],) >>>> spam[0] is ham > True >>>> spam[0] += [5] > Traceback (most recent call last): > File "<stdin>", line 1, in <module> > TypeError: 'tuple' object does not support item assignment >>>> ham += [5] >>>> spam > ([1, 2, 3, 4, 5, 5],) >>>> > > What do you say to that? > > I can come up with much mumbling about pointers and stacks and > heaps and much hand-waving about the underlying this-and-that, but > nothing that sounds even remotely illuminating. > > Your suggestions would be much appreciated!
You might find the following helpful (partially): http://effbot.org/zone/call-by-object.htm Cheers, Chris -- http://blog.rebertia.com -- http://mail.python.org/mailman/listinfo/python-list
