Something in the function tutorial confused me.

[Roel Schroeven]

Gregory D. Weber schreef:
> Neil Cerutti wrote:
>> On 2007-08-11, Alex Martelli <aleax at mac.com> wrote:
>>> Neil Cerutti <horpner at yahoo.com> wrote:
>>>    ...
>>>> The Python Language Reference seems a little confused about the
>>>> terminology.
>>>>
>>>>   3.4.7 Emulating numeric types
>>>>   6.3.1 Augmented assignment statements
>>>>
>>>> The former refers to "augmented arithmetic operations", which I
>>>> think is a nice terminology, since assignment is not necessarily
>>>> taking place. Then the latter muddies the waters.
>>> Assignment *IS* "necessarily taking place"; if you try the augmented
>>> assignment on something that DOESN'T support assignment, you'll get an
>>> exception.  Consider:
>>>
>>>>>> tup=([],)
>>>>>> tup[0] += ['zap']
>>> Traceback (most recent call last):
>>>   File "<stdin>", line 1, in <module>
>>> TypeError: 'tuple' object does not support item assignment
>>>
>>> Tuples don't support item ASSIGNMENT, and += is an ASSIGNMENT,
>>> so tuples don't allow a += on any of their items.
>>>
>>> If you thought that += wasn't an assignment, this behavior and
>>> error message would be very problematic; since the language
>>> reference ISN'T confused and has things quite right, this
>>> behavior and error message are perfectly consistent and clear.
>> Thanks for the correction. I was under the illusion that
>> sometimes augmented assignment would instead mutate the
>> object.
>>
> 
> I too thought += was an assignment.  And it bit me very painfully a few weeks ago.

It is an assignment, but not only an assignment: first either the 
creation of a new object (in case of immutable objects) or the 
modification of an existing object (in case of mutable objects), than an 
assignment.

> If it's an assignment, then wouldn't "x += y" mean the same thing as "x = x + y"?

No:
- x += y, x is only evaluated once
- in x += y, the operation is performed in-place if possible, meaning 
that (quoting from the language reference): rather than creating a new 
object and assigning that to the target, the old object is modified instead

> If so, why does an assignment to variable a, below, have the *side effect* of changing variable b ... ?
> 
>>>> a = [1, 2, 3]
-> A list is created and assigned to a
>>>> b = a
-> That list is also assigned to b
>>>> b
> [1, 2, 3]
>>>> a += [4]
-> The list is modified (the list [4] is added to it) and assigned to a. 
It was already assigned to a, so that assignment doesn't do very much in 
this case. But it is performed.
>>>> a
> [1, 2, 3, 4]
>>>> b
> [1, 2, 3, 4]
-> Both a and b are still bound to the original (but now modified) list 
object

> ... but using the "x = x + y" style, the assignment to variable c, below, does *not* have a side effect on variable d (as indeed it should not!)?
> 
>>>> c = [1, 2, 3]
-> A list is created and assigned to c
>>>> d = c
-> That list is also assigned to d
>>>> d
> [1, 2, 3]
>>>> c = c + [4]
-> A new list object is created (the sum of the original one and the 
list [4]) and assigned to c
>>>> c
> [1, 2, 3, 4]
-> c is now bound to the new list object
>>>> d
> [1, 2, 3]
-> ... while d is still bound to the original list object


I hope this clears up things a bit...

-- 
If I have been able to see further, it was only because I stood
on the shoulders of giants.  -- Isaac Newton

Roel Schroeven