Re: problem with logical and

On 7/06/2013 1:31pm, Mike Dewhirst wrote:
> I can't work this one out. I have a blind spot. Any help appreciated.

Accidentally fixed this trying workarounds. Still not sure what was
happening but I suspect I don't fully understand None in the boolean
context. Anyway, I'm being more explicit now - with some success!

Sorry to bother you ...

Mike

>
> My unit tests indicate a problem with the following method (is_current)
> on a model. Because I couldn't figure it out I instrumented with a whole
> mess of print statements. The problem occurs on the line with
> self.quality_tag. Note the print statement before and after ...
>
> def is_current(self):
> current = self.effective and self.effective <= when()
> print('\n%s %s - when = %s' %(current, self.effective, when()))
>
> print('\n%s - 1 quality_tag = %s' % (current, self.quality_tag))
> current = current and (self.quality_tag is None)
> print('\n%s - 2 quality_tag = %s' % (current, self.quality_tag))
>
> current = current and not self.withdrawn
> print('\n%s - withdrawn = %s' %(current, self.withdrawn))
> return current
>
> Here is the unit test output ... which seems to indicate there is a bug
> in (my) boolean logic where (self.quality_tag is None) returns False
> instead of True when self.quality_tag really *is* None
>
> True 2013-06-07 03:18:14.418000+00:00 - when = 2013-06-07
> 03:18:15.231000+00:00
>
> True - 1 quality_tag = None
>
> False - 2 quality_tag = None
>
> False - withdrawn = False
> F
> ======================================================================
> FAIL: test_attach_current (items.tests.test_utils.TestAttach)
> ----------------------------------------------------------------------
> Traceback (most recent call last):
> File "C:\users\miked\py\xxx_project\xxx\items\tests\test_utils.py",
> line 272, in test_attach_current
> self.assertEqual(obj.is_current(), True)
> AssertionError: False != True
>
> ----------------------------------------------------------------------
>
> Windows XP
> Python 2.7
> Django 1.5
>
> Thanks
>
> Mike
>

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