I like the idea, but there seems to be a bit of a performance hit. >>> from timeit import Timer >>> mokatimer = Timer("l = List(range(1000)).extend(range(1000

I like the idea, but there seems to be a bit of a performance hit. >>> from timeit import Timer >>> mokatimer = Timer("l = List(range(1000)).extend(range(1000, 2000)).keep(lambda x: (x % 3) == 1)", setup="from moka import List") >>> regtimer = Timer("l = range(1000); l.extend(range(1000, 2000)); l = [n for n in l if (n % 3) == 1]") >>> mokatimer.timeit(10000) 16.502125024795532 >>> regtimer.timeit(10000) 2.5419421195983887 I tried pre-generating the range to see if that was throwing it off the timer: >>> regtimer = Timer("l = []; l.extend(range1000); l = [n for n in l if (n % 3) == 1]", setup="range1000 = range(1000)") >>> mokatimer = Timer("l = List().extend(range1000).keep(lambda x: (x % 3) == 1)", setup="from moka import List; range1000=range(1000)") >>> regtimer.timeit(10000) 1.0892798900604248 >>> mokatimer.timeit(10000) 7.714169979095459 I didn't look at the source, but it looks like moka needs to be optimized to use list comprehension for .keep. Other methods could probably use comprehension (or generators) too.