returning True, False or None

[Brian van den Broek]

Fahri Basegmez said unto the world upon 2005-02-04 23:14:
> "Mick Krippendorf" <mad.mick at gmx.de> wrote in message 
> news:36iv03F4urakqU1 at individual.net...
> 
>>Fahri Basegmez wrote:
>>
>>>reduce(lambda x, y: x or y, lst)
>>
>>This doesn't solve the OPs problem since
>>
>>
>>>>>reduce(lambda x, y: x or y, [False, None])
>>
>>returns None instead of False.
>>
>>Mick.
>>
> 
> 
> You are right.
> I tested None or False and it worked. I assumed order did not matter for or 
> operator.
> 
> None or False returns False
> False or None returns None
> 
> You know what they say about assumptions. Live and learn.
> 
> Fahri

Hi Fahri,

I don't have a reference at hand, but you might want to check the 
docs' index or do a google for short circuit python or something similar.

or works by evaluating the first value and returning it if it 
evaluates to True. Otherwise it returns the second.
 >>> 0 or 42
42
 >>>

Likewsie, and returns the first if it evaluates to False, otherwise it 
returns the second.
 >>> [] and 42
[]
 >>>

The idea is that the evaluation breaks out as soon as it has seen 
enough to determine the result. Hence, short circuit. And, instead of 
returning a Boolean, it returns the actual object flanking the 
operator. Hence, the behaviour observed.

HTH,

Brian vdB