None or 0
tim.one at comcast.net
Sat Sep 7 13:11:10 EDT 2002
> I'm using the idiom
> a = a or b
> to get rid of statements like
> if a is None:
> a = b
That's fine, although you may get burned someday if 'a' has an interesting
value that's not None but happens to evaluate as false. Like, e.g.,
# Default to 1 pound.
to_eat = number_of_pounds or 1
If you're not hungry and try to do eat(0), you're in for a treat. As a rule
of thumb, whenever you use None to mean "not there", you'll cut bugs by
testing for "is None" or "is not None" explicitly. I've seen many subtle
bugs as a result of not doing this; sometimes code works fine for years, and
then someone happens to add a __len__ method to an object's class, and "all
of a sudden" evaluating to false no longer means a var is still bound to
None, but that it may also be bound to a non-None object that suddenly
decided to call itself empty.
> Now I wonder what happens if both a and b have zero-length values.
If A evaluates to false, 'A or B' returns B without testing B (so it doesn't
matter whether B does or doesn't evaluate to false -- B is simply evaluated
and returned then).
> Trying in Python 2.2.1 reveals:
> >>> repr(0 or None)
> >>> repr(None or 0)
> >>> repr('' or None)
> >>> repr(None or '')
> Is it guaranteed to work like this
Yes. Similarly, 'A and B' returns B untested whenever A evaluates to true.
A or B same-as if A then A else B
A and B same-as if A then B else A
B is never tested for true/false, and in the cases where A is the result the
B expression isn't even evaluated.
> or should that be avoided?
That's really a different question <wink>.
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