Relax Syntax for Augmented Arithmetic?

Steven D'Aprano steven at REMOVE.THIS.cybersource.com.au
Mon Jan 19 04:36:01 CET 2009


On Sun, 18 Jan 2009 15:11:46 -0500, Terry Reedy wrote:

> andrew cooke wrote:
>> Context -
>> http://docs.python.org/3.0/reference/datamodel.html?highlight=data
>> model#object.__iadd__
>> 
>> Just a suggestion I thought I'd throw out...  There's a restriction in
>> the language implementation on exactly what can go the left of an
>> augmented arithmetic expression.
>> 
>> For example:
>>>>> a = 3
>>>>> a **= 2
>> 
>> is ok, but:
>>>>> class Foo():
>> ...   def __init__():
>> ...     self.a = 3
>> ...   def __ipow__(self, x):
>> ...     self.a **= x
>> ...
>>>>> Foo() **= 2
> 
> Calls return objects and therefore cannot be the target of an
> assignment, augmented or otherwise.  The target of an assignment is a
> name or collection slot, both of which are grammatical constructs, not
> objects.

There's a practical reason too. You create a new Foo instance, mutate it 
with the augmented assignment operator, and then a tenth of a millisecond 
later the garbage collector throws it away because it has a reference 
count of zero.



-- 
Steven



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