filter() and booleanness

[Peter Hansen]

rihad wrote:
> 
> Hi there, the following two calls will both print ('hi', 'there') due
> to None being substituted for `identity function' (as the docs say):
> 
> print filter(None, ('hi', None, 0, 'there'))
> print filter(lambda x: x, ('hi', None, 0, 'there'))
> 
> Then I thought I'd clamp the x to [0..1] but things like x != 0 missed
> None and x != None missed 0. :) Now I have lambda x: not not x, is it
> bulletproof? (it seems to work).

I believe I understand what you're asking :-), and yes, 
it _is_ bulletproof.  That's an idiom for mapping false
to 0 and true to 1, regardless of the original values.