On Jul 7, 4:04 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]
You have transposed some_list and some some_tuple. I.e. you should be calling eggs(x, z, y). > How do I explain to rank beginners (no programming experience at > all) why x and y remain unchanged above, but not z? You don't. Rank beginners don't have enough background knowledge to grok that code. Why would you even tell the poor bastards about "+=" before they were comfortable with (python's style of) function calls, immutable integers, mutable lists and immutable tuples? Let them use "x = x + y" until they have enough knowledge to understand "augmented" assignment. Syntax matters (I mean general order of things, not linguistic syntax.) Making an omelette requires putting eggs in a pan and cracking them, but not in that order. > 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 say, "Don't use augmented assignment with indexed tuples." Seriously, python's augmented assignment is almost magical. I think you're just making trouble for yourself and your students if you introduce it too early. I get python pretty well (if I say so myself) but I wouldn't know how to explain: In [1]: def foo(a_list): ...: a_list = a_list + [5] ...: ...: In [2]: n = [] In [3]: foo(n) In [4]: n Out[4]: [] In [5]: def bar(a_list): ...: a_list += [5] ...: ...: In [6]: bar(n) In [7]: n Out[7]: [5] It's "Just The Way It Is". Freakin' magic, man. > 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! Frankly, I'm of the impression that it's a mistake not to start teaching programming with /the bit/ and work your way up from there. I'm not kidding. I wrote a (draft) article about this: "Computer Curriculum" http://docs.google.com/View?id=dgwr777r_31g4572gp4 I really think the only good way to teach computers and programming is to start with a bit, and build up from there. "Ontology recapitulates phylogeny" I realize that doesn't help you teach python, but I wanted to put it out there. -- http://mail.python.org/mailman/listinfo/python-list
