[Numpy-discussion] "False" and "True" not singletons?
Tue Dec 30 13:59:43 CST 2008
I've just discovered that "False" is not a singleton:
>>> import numpy as N
>>> f = N.all((1,2,0))
>>> print f
>>> f is False
Should it be?
This arose for me in some tests I'm using that check if a result is False:
self.failUnless ( (self.B1 == self.B3) is False )
I'm doing it this way, because I want to make sure that the __eq__
method really is returning "True" or "False", rather than a value that
happens to evaluate to true or false, like 1, or an empty list, or whatever.
This is interesting to me because back when Python first introduced
Booleans, I had thought they should be kept pure and not be subclasses
of integers, and, in fact, "if" should only except boolean values.
However this opinion was really a matter of my sense of purity, and this
is the fist time I've run into a case that matters.
I suppose I'm not being pythonic -- I should really only care if the
result evaluates true or false, but I don't feel like I'm testing right
if values can slip though that shouldn't.
It does reinforce my opinion though -- whether zero, or an empty
sequence or string should evaluate false really is a matter of specific
application, not universal.
Anyway, should I just give up? Or should numpy return the same "True"
and "False" (like None), or is there another solution?
x == False
is close, but zero still slips through, even though an empty list does not:
>>> 0 == False
>>>  == False
Christopher Barker, Ph.D.
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