Re: [Pypgsql-devel] [ pypgsql-Bugs-1755777 ] PgNumeric += is strange

[Billy G. A. <bil...@de...>]

Billy G. Allie wrote:
> SourceForge.net wrote:
>> Bugs item #1755777, was opened at 2007-07-17 18:56
>> Message generated for change (Tracker Item Submitted) made by Item 
>> Submitter
>> You can respond by visiting: 
>> https://sourceforge.net/tracker/?func=detail&atid=116528&aid=1755777&group_id=16528 
>>
>>
>> Please note that this message will contain a full copy of the comment 
>> thread,
>> including the initial issue submission, for this request,
>> not just the latest update.
>> Category: PgNumeric
>> Group: None
>> Status: Open
>> Resolution: None
>> Priority: 5
>> Private: No
>> Submitted By: Ken Lalonde (kenlalonde)
>> Assigned to: Nobody/Anonymous (nobody)
>> Summary: PgNumeric += is strange
>>
>> Initial Comment:
>> The augmented arithmetic operations on PgNumeric instances will 
>> update the object directly,
>> instead of creating a new one.
>> This leads to unexpected results such as:
>>
>> $ cat t.py
>> from pyPgSQL.PgSQL import PgNumeric
>> a = PgNumeric(1.0)
>> b = a
>> b += 1
>> print a, b
>> $ python t.py
>> 2.0 2.0
>>
>> Here we see b is incremented, but both a and b
>> are changed, because the __iadd__ method updates
>> b in place.
>>
>> Call me naive, but I would expect "b += 1"
>> to behave exactly like "b = b+1".
>>
>> Perhaps PgNumeric should allocate a new object
>> if the old one has multiple references?
>> I'm not sure, but the current behavior
>> is not intuitive, and leads to hard-to-find bugs.
> It's in the nature of the in-place operators when they apply to 
> objects. The statement "b +=1" adds one to the existing object 
> referenced by
> "b". Since a is also a reference to the same object, it also displays 
> the new result. The statement "b = b + 1" creates a new object with 
> the value of "b + 1" and puts a reference to it into b.
>
> Ok, that explains the problem. The question is how to fix it. The 
> solution as you pointed out would be to create a new PgNumeric object 
> and place a reference to it into "b", but I don't know if it is 
> possible to do that. Any thoughts on the problem would be appreciated.
>
> -- 
> Bill Allie
>
>
It turned out to be easier to fix then I thought. Attached is a patch to 
fix the problem.
--
Bill Allie.