Check if variable is defined
gaul at spam.utexas.edu
Mon May 21 04:04:23 CEST 2001
In article <mailman.990091708.9966.python-list at python.org>, Todd A.
> On Thu, 17 May 2001, Tim Peters wrote:
>> if beatsme is None:
>> print "beatsme wasn't assigned anything meaningful yet"
> This works. It also turns out that:
> if foo: print "true" # print true if foo not empty
> works too, which is the behavior I was looking for. It doesn't work
> quite so well on numbers (your example works better), but is fine for
> lists, which is what I really needed anyway: something to detect empty
More generally, many values are considered false: None, 0, and an empty
map or sequence. From a clarity point of view, 'is None' is often the
best. It's also the fastest, as the 'is' keyword only checks for object
identity. I think that None has the same sort of implementation trick
as small integers (it returns the same id()).
One can find more information about truth values here:
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