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

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

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