[Tutor] question concerning Boolean operators
steve at alchemy.com
Fri Mar 21 01:02:10 CET 2008
On Thu, Mar 20, 2008 at 04:55:17PM -0700, jim stockford wrote:
> >>> string1, string2, string3 = '', 'Trondheim', 'Hammer Dance'
> >>> jackson = string1 or string2 or string3
> >>> jackson
The key here is the "or" operator which, in an expression like
a or b
will return a if a is a True boolean value or b otherwise. The upshot
is that the result will be True if a OR b is True, and False only if they
are both False. The *actual* value returned will be the value of a or b.
so here we have:
string1 or string2 or string3
If string1 is True (which a string is if it's not empty), then that's
the result returned, and we can stop right there.
non_null = '' or 'Trondheim' or 'Hammer Dance'
However, string1 ('') is empty, which in a Boolean context is False,
so Python goes on to string2 ('Trondheim') That's non-empty (or True)
so we get that value and don't look further.
non_null = 'Trondheim'
which is a True value. If you said:
print non_null, 'is not null'
print "it's null"
The result would be to print:
Trondheim is not null
Hope that helps
> On Mar 20, 2008, at 4:32 PM, Guba wrote:
> > Dear list,
> > from Guido's tutorial:
> > It is possible to assign the result of a comparison or other Boolean
> > expression to a variable. For example,
> > >>> string1, string2, string3 = '', 'Trondheim', 'Hammer Dance'
> > >>> non_null = string1 or string2 or string3
> > >>> non_null
> > 'Trondheim'
> > How does this work?? How does Python know that we are looking for
> > non_null? After all, we don't provide this information here, right? (I
> > take non_null is a variable.)
> > I must be missing something... Can anyone help?
> > Many thanks,
> > Guba
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