returning True, False or None

[Jeremy Bowers]

On Fri, 04 Feb 2005 10:48:44 -0700, Steven Bethard wrote:

> I have lists containing values that are all either True, False or None, 
> e.g.:
> 
>      [True,  None,  None,  False]
>      [None,  False, False, None ]
>      [False, True,  True,  True ]
>      etc.
> 
> For a given list:
> * If all values are None, the function should return None.
> * If at least one value is True, the function should return True.
> * Otherwise, the function should return False.
> 
> Right now, my code looks like:
> 
>      if True in lst:
>          return True
>      elif False in lst:
>          return False
>      else:
>          return None

Yes, I see the smell, you are searching the list multiple times. You
could bail out when you can:

seenFalse = False
for item in list:
	if item: return True
	if item is False: seenFalse = True
if seenFalse:
	return False
return None

But I'd submit that if four item lists are your common case, that your
original code is significantly easier to understand what it is doing. This
can be alleviated with an appropriate comment on the chunk of code I gave
you, though.