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

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Bugs item #1755777, was opened at 2007-07-18 00:56
Message generated for change (Comment added) made by bpietro2
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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.



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Comment By: Pietro Bernardi (bpietro2)
Date: 2008-03-09 15:40

Message:
Logged In: YES 
user_id=2014513
Originator: NO

IMHO you are right, it's worse, this bug affects not only '+='. See this:

>>> ZERO = PgSQL.PgNumeric('0')
>>> a = ZERO; b = ZERO; b = b + 1; c = ZERO
>>> print ZERO,a,b,c
0 0 1 0
>>> a = ZERO; b = ZERO; b += 1; c = ZERO
>>> print ZERO,a,b,c
1 1 1 1
>>> ZERO = PgSQL.PgNumeric('0')
>>> a = ZERO; b = ZERO; b = b - 1; c = ZERO
>>> print ZERO,a,b,c
0 0 -1 0
>>> a = ZERO; b = ZERO; b -= 1; c = ZERO
>>> print ZERO,a,b,c
-1 -1 -1 -1
>>> TWO = PgSQL.PgNumeric('2')
>>> a = TWO; b = TWO; b = b * 3; c = TWO
>>> print TWO,a,b,c
2 2 6 2
>>> a = TWO; b = TWO; b *= 3; c = TWO
>>> print TWO,a,b,c
6 6 6 6        
>>> TWO = PgSQL.PgNumeric('2')
>>> a = TWO; b = TWO; b = b / 2; c = TWO
>>> print TWO,a,b,c
2 2 1 2
>>> a = TWO; b = TWO; b /= 2; c = TWO
>>> print TWO,a,b,c
1 1 1 1

Regards,
Pietro

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