Something in the function tutorial confused me.

Gregory D. Weber spottedMetal at gmail.com
Sat Aug 11 22:45:10 CEST 2007


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.

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

If so, why does an assignment to variable a, below, have the *side effect* of changing variable b ... ?

>>> a = [1, 2, 3]
>>> b = a
>>> b
[1, 2, 3]
>>> a += [4]
>>> a
[1, 2, 3, 4]
>>> b
[1, 2, 3, 4]

... 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]
>>> d = c
>>> d
[1, 2, 3]
>>> c = c + [4]
>>> c
[1, 2, 3, 4]
>>> d
[1, 2, 3]
>>> 



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