list partition

[Moosebumps]

Is there a function that takes a list and predicate, and returns two
lists -- one for which the predicate is true and one for which it is false?

I know I can call filter(p, list) and filter( not p, list) or something like
that, but I assume it would be more efficient to do it in one pass.

Actually I probably can't call "not p", I would have to wrap p in a function
notP or something. Or is there a shortcut for lambda(x) : not p(x)? I
guess it isn't that long : )

MB

 

[David M. Cooke]

Is there a function that takes a list and predicate, and returns two
lists -- one for which the predicate is true and one for which it is false?

I know I can call filter(p, list) and filter( not p, list) or something like
that, but I assume it would be more efficient to do it in one pass.

Not too hard to write your own:

def filtersplit(p, iterable):
ptrue = []
pfalse = []
for x in iterable:
if p(x):
ptrue.append(x)
else:
pfalse.append(x)
return ptrue, pfalse

Actually I probably can't call "not p", I would have to wrap p in a function
notP or something. Or is there a shortcut for lambda(x) : not p(x)? I
guess it isn't that long : )

itertools.ifilterfalse negates the condition for you.

 

[Peter Otten]

Is there a function that takes a list and predicate, and returns two
lists -- one for which the predicate is true and one for which it is
false?

I take a slightly different approach, which is not limited to boolean
predicates:
.... d.setdefault(predicate(i), []).append(i)
....

d[True] [1, 3, 5, 7, 9]
d[False] [0, 2, 4, 6, 8]

Peter