Python dot-equals (syntax proposal)

[Steven D'Aprano]

On Mon, 03 May 2010 06:37:49 +0200, Alf P. Steinbach wrote:

> * Terry Reedy:
>> * Alf P. Steinbach:
>>> * Aahz:
>>
>>>> and sometimes
>>>> they rebind the original target to the same object.
>>>
>>> At the Python level that seems to be an undetectable null-operation.
>>
>> If you try t=(1,2,3); t[1]+=3, if very much matters that a rebind
>> occurs.
> 
> Testing:
> 
> <test lang="py3">
>  >>> t = ([], [], [])
>  >>> t
> ([], [], [])
>  >>> t[0] += ["blah"]
> Traceback (most recent call last):
>    File "<stdin>", line 1, in <module>
> TypeError: 'tuple' object does not support item assignment
>  >>> t
> (['blah'], [], [])
>  >>> _
> </test>
> 
> Yep, it matters.
> 
> Is this change-but-raise-exception a bug?
> 
> I seem to have a knack for running into bugs. :-)

No, I don't believe so -- I believe that it is behaving exactly as 
advertised. But it is absolutely a gotcha.

Consider:


>>> class K(object):
...     def __init__(self, value=0):
...             self.value = value
...     def __add__(self, other):
...             self.value = self.value + other
...             return self
...     def __str__(self):
...             return "%s" % self.value
...     __repr__ = __str__
...
>>> x = K(42)
>>> x + 5
47
>>> t = (None, x)
>>> t
(None, 47)
>>>
>>> t[1] + 3
50
>>> t
(None, 50)
>>> t[1] += 1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'tuple' object does not support item assignment
>>> t
(None, 51)


Unintuitive, yes. Possibly a bad design, maybe. Surprising, absolutely. 
But not a bug, as it's working exactly as promised. += is conceptually 
two steps: perform an addition, and perform an assignment afterward. That 
addition is sometimes performed in-place, but regardless of whether it is 
or not, the assignment is always attempted.



-- 
Steven