list_a-list_b

[Raymond Hettinger]

[David Eppstein]
> > Better is to convert b to a dictionary
> > (where are dict comprehensions when you need them):
> >
> >     dict_b = {}
> >     for x in list_b: dict_b[x] = 1

[Tim Peters]
> Looks clear enough to me, although
>
>     for x in list_b:
>         dict_b[x] = 1
>
> is clearer.  If you like, you can do this:
>
>     dict([(x, 1) for x in list_b])
>
> or this:
>
>     dict(zip(list_b, [1]*len(list_b)))
>
> or even:
>
>     dict(zip(list_b, list_b))

My favorites:

dict(map(None, list_b, []))   # doesn't loop over list_b twice

or

dict_b = {}
map(dict_b.__setitem__, b, [])   # fastest for Py2.2


Raymond Hettinger