weird behaviour of "0 in [] is False"

Sylvain Thenault sylvain.thenault at nospam.logilab.fr
Tue Nov 30 14:32:59 CET 2004


On Tue, 30 Nov 2004 14:18:30 +0100, Diez B. Roggisch wrote:

>>>>> l = []
>>>>> 0 in (l is False)
>> Traceback (most recent call last):
>>   File "<stdin>", line 1, in ?
>> TypeError: iterable argument required
> 
> that should be clear - 0 in False can't work.

yes, I forget to mention that it was the third expression which was
bugging me.

>>>>> (0 in l) is False
>> True
>>>>> 0 in l is False
>> False
> 
> It seems to stem from a behaviour python exhibits in expressions like
> this:
> 
> 3 > 2  > 1
> 
> This yields True, while
> 
> (3 > 2 ) > 1
> 
> yields false. Chaining of operators gets translated like this:
> 
> 3 > 2 and 2 > 1
> 
> Look in section 5.9 of the language reference.
> 
> Then your expression gets translated to:
> 
> 0 in l and l is False
> 
> which yields False of course.

thanks, I had missed this part of the language reference !
Not yet found an misbehaviour in python... ;)

-- 
Sylvain Thénault                               LOGILAB, Paris (France).

http://www.logilab.com   http://www.logilab.fr  http://www.logilab.org





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