checking if two things do not equal None

Steven D'Aprano steve+comp.lang.python at pearwood.info
Sun Mar 30 08:08:37 CEST 2014


On Sat, 29 Mar 2014 19:54:09 -0700, Rustom Mody wrote:

> On Sunday, March 30, 2014 8:09:45 AM UTC+5:30, Roy Smith wrote:
>> I have no particular problem with
> 
>>  x < 2 < y
> 
>> because it fits the same pattern.  But, if you show me
> 
>>  a != None != b:
> 
>> my brain just goes into overload.  Honestly, I don't even know what
>> that means.  My brain keeps trying to stick a, None, and b on Mrs.
>> Albaum's number line and keeps walking into the wall.  If you (the
>> editorial you) tell me that my failure to grok that expression means
>> I'm not fluent in Python, well then, guilty as charged.
> 
> <Math Terminology>
[...]
> So for != chained comparisons are not natural (or IMHO appropriate)

I tend to agree they're "not natural", although appropriate is another 
thing. The problem is that we tend to read something like:

    a != b != c

as "all of a, b and c are unequal", corresponding to:

    a == b == c

as "all of a, b and c are equal". But that's not what it means. It means 
that a != b and b != c, but it says nothing about a and c. And that was 
my mistake. The OP actually got it right in their first post, but 
sticking None in the middle to ensure it partakes of both comparisons.

    a is not None is not b

Still, that's not easily extended to a third item, this would be wrong:

    a is not None is not b is not c

since c only gets compared against b, not None. Better is to factor the 
"not" out:

    not (a is b is c is None)


which now should be clear: you're testing whether or not *all* of a, b 
and c are None. If you prefer:

    not all(x is None for x in (a, b, c))


Which is more readable is a matter of personal preference.

I think Johannes got it right: boolean logic is easier to reason about 
when there is a minimum of "not"s.


-- 
Steven D'Aprano
http://import-that.dreamwidth.org/



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