tough-to-explain Python

[Francesco Bochicchio]

On Jul 7, 10:04 pm, kj <no.em... at 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!
>
> TIA!
>
> kj

I would go with something like this:

"""
In object oriented programming, the same function or operator can be
used to represent
different things. This is called overloading. To understand what the
operator/function do, we have to look at
the kind of object it is applied to.
In this case, the operator "+=" means two different things:
- for strings and numbers it means : "create a new object by merging
the two operands". This is why the original object is left the same.
- for lists, it means : "increase the left operand with the contents
of the right operand". This is why the original object is changed
"""

You couuld also add:
"""
You see, in python everithing is an object. Some object can be changed
(mutable objects), others cannot.
"""
but this is another story.


P:S : Sometime I think they should not have allowed += on immutables
and forced everybody to write s = s + "some more".

Ciao
----
FB