tough-to-explain Python

[Piet van Oostrum]

>>>>> kj <no.email at please.post> (k) wrote:

>k> I'm having a hard time coming up with a reasonable way to explain
>k> certain things to programming novices.

>k> Consider the following interaction sequence:

>>>>> def eggs(some_int, some_list, some_tuple):
>k> ...     some_int += 2
>k> ...     some_list += [2]
>k> ...     some_tuple += (2,)
>k> ...
>>>>> x = 42
>>>>> y = (42,) 
>>>>> z = [42] 
>>>>> eggs(x, y, z)
>>>>> x
>k> 42
>>>>> y
>k> (42,)
>>>>> z
>k> [42, 2] 
>>>>> 

>k> How do I explain to rank beginners (no programming experience at
>k> all) why x and y remain unchanged above, but not z?

You shouldn't. That's not for beginners. Leave it waiing until you get
to the advanced level.

>k> Or consider this one:

>>>>> ham = [1, 2, 3, 4]
>>>>> spam = (ham,)
>>>>> spam
>k> ([1, 2, 3, 4],)
>>>>> spam[0] is ham
>k> True
>>>>> spam[0] += [5]
>k> Traceback (most recent call last):
>k>   File "<stdin>", line 1, in <module>
>k> TypeError: 'tuple' object does not support item assignment
>>>>> ham += [5]
>>>>> spam
>k> ([1, 2, 3, 4, 5, 5],)
>>>>> 

>k> What do you say to that?

Mutable and immutable. But use different examples. Like

ham = [1, 2, 3, 4]
spam = (1, 2, 3, 4)

spam[0] += 1 will give the same error message. You can't change the
components of a tuple.

Your example above is similar. The spam[0] += [5] appends the 5 to the
list in spam[0] (so it appends to ham), and then tries to assign the
result of it to spam[0], which is not allowed. That the item it tries to
assign is the same as the item that was already there doesn't matter.

So dont't forget += is a real assignment, even when it is an in-place
modification.  Your example just proves that. The language ref manual
says:

    With the exception of assigning to tuples and multiple targets in a
    single statement, the assignment done by augmented assignment
    statements is handled the same way as normal assignments.

But I think that your example isn't for beginners either.
-- 
Piet van Oostrum <piet at cs.uu.nl>
URL: http://pietvanoostrum.com [PGP 8DAE142BE17999C4]
Private email: piet at vanoostrum.org