On Sun, Aug 16, 2009 at 12:24 PM, bobhaugen <bob.hau...@gmail.com> wrote:
>
> 1, I ran into this anomaly in updating a decimal field:
>
> The field:
>
> remaining = models.DecimalField(max_digits=8, decimal_places=2,
> default=Decimal("0"))
>
> The code that failed:
>
> 658 self.remaining -= qty
> 659 self.remaining = max([Decimal("0"), self.remaining])
> 660 self.save()
>
> The error message:
>
> TypeError
> unsupported operand type(s) for -=: 'unicode' and 'Decimal'
> models.py in update_from_transaction, line 658
>
> The workaround:
>
> remaining = Decimal(self.remaining)
> remaining -= qty
> self.remaining = max([Decimal("0"), remaining])
> self.save()
>
> So, what gives?
>
> Is this an example of this bug?
> http://code.djangoproject.com/ticket/5903
>
> (P.S. the object being updated had been saved before the code in error
> ran. In other words, the error happened in a re-update of a
> previously saved object.)
>
Might be, but hard to say for sure based on the information. For that bug
it does not matter if the model instance has been saved (though the first
sentence of the description may mislead one to think that), it matters how
the Python object you are working with was created, and you didn't tell us
that. If you retrieved it from the database (assuming DThing is the model
containing the field remaining):
dt = DThing.objects.get(pk=n)
then dt.remaining should be a DecimalField, with or without the fix for that
bug.
If the Python object was created on the fly and relied on the default value:
dt1 = DThing()
dt1.save()
or:
dt2 = DThing.objects.create()
Then prior to the fix for that bug, dt1.remaining and dt2.remaining would be
unicode strings instead of Decimal objects. The fix for #5903 corrects
things so that remaining is a Decimal value in this case.
If the Python object was created on the fly and specified remaining as a
unicode string:
dt3 = DThing.objects.create(remaining=u'44')
then dt3.remaining will be a unicode string, since that is what was passed
in during creation. (Same if you use the sequence of calls used for dt1
instead of using create.) This is the point mentioned here:
http://code.djangoproject.com/ticket/5903#comment:10
This behavior is not affected by the fix for #5903.
>
> 2. This changeset says it has fixed ticket #5903:
> http://code.djangoproject.com/changeset/9823
>
> Also says it was fixed 6 months ago. I did not find the ticket listed
> in any of the milestones in the last 6 months, and the page does not
> list a milestone. Does that mean it has not been included in any
> release, and awaits 1.2?
>
No. The milestone field is just a way of tagging work to be done, it has no
effect on what changesets go into a release. All changesets applied to
trunk prior to the 1.1 release are part of 1.1, thus r9823, which was
applied to trunk in Feb., is included in 1.1.
Since this sounds like a bug fix, it should also have been applied to the
1.0.X branch as well, but the ticket doesn't note that. Unfortunately the
post-commit hook that automatically adds those notes sometimes fails to
trigger (I have no idea why) so they are sometimes missing and you have to
manually check to see if a particular changeset was backported to a branch.
In this case just hitting "Next changeset" on that changeset's page shows
that this fix was also applied to the 1.0.X branch:
http://code.djangoproject.com/changeset/9824
Thus the fix is also available in the first release of 1.0.X made after Feb.
09, which is 1.0.3.
>
> 3. Also, what is the best practice for specifying a DecimalField
> default? I did some googling and found code using default="0",
> default=Decimal(0), default=Decimal("0"), and default=Decimal("0.0").
> I did some tests and they all seem to work.
>
Best practice is for it to be a Decimal object, not a string. Beyond that I
don't care to differentiate though personally I'd attempt to be consistent
and not mix passing in integers and strings and strings with trailing
fractional zero parts willy-nilly.
Karen
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