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From: Peter Otten <__peter__@web.de>
Subject: Re: numpy.where() and multiple comparisons
Date: Sat, 18 Jan 2014 09:50 +0100
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Xref: csiph.com comp.lang.python:64220

John Ladasky wrote:

> On Friday, January 17, 2014 6:16:28 PM UTC-8, duncan smith wrote:
> 
>>  >>> a = np.arange(10)
>>  >>> c = np.where((2 < a) & (a < 7))
>>  >>> c
>> (array([3, 4, 5, 6]),)
> 
> Nice!  Thanks!
> 
> Now, why does the multiple comparison fail, if you happen to know?

2 < a < 7

is equivalent to

2 < a and a < 7

Unlike `&` `and` cannot be overridden (*), so the above implies that the 
boolean value bool(2 < a) is evaluated. That triggers the error because the 
numpy authors refused to guess -- and rightly so, as both implementable 
options would be wrong in a common case like yours.

(*) I assume overriding would collide with short-cutting of boolean 
expressions.