Re: [Numpy-discussion] comparing string array with None raises ValueError

24 Jun 2004


      On June 24, 2004 01:46 pm, Sebastian Haase wrote:
...
In general it must be OK to compare anything with None, right ?
(BTW, I get the same error with == and !=)
No! Not in general! I learnt this back when Numeric implemented rich 
comparisions; suddenly, lots of my code broke. You don't actually want "is 
this object _equal_ (or not equal) to None", you want "is this object None", 
as None is a singleton.
A contrived example:
...
...
...
class A:
...     def __eq__(self, other): return True
...
...
...
...
a = A()
So, this is bad:
...
...
...
a == None
True
This is good:
...
...
...
a is None
False
In general, if you're testing if something is None, use 'is', not '=='.
...
...
...
q.Mrc.hdr('title') is not None
-- 
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|David M. Cooke                      http://arbutus.physics.mcmaster.ca/dmc/
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