Split list into multiple lists based on indices [duplicate]

Question

What is the best way to split a list into parts based on an arbitrary number of indexes? E.g. given the code below

indexes = [5, 12, 17]
list = range(20)

return something like this

part1 = list[:5]
part2 = list[5:12]
part3 = list[12:17]
part4 = list[17:]

If there are no indexes it should return the entire list.

Answer 1

This is the simplest and most pythonic solution I can think of:

def partition(alist, indices):
    return [alist[i:j] for i, j in zip([0]+indices, indices+[None])]

if the inputs are very large, then the iterators solution should be more convenient:

from itertools import izip, chain
def partition(alist, indices):
    pairs = izip(chain([0], indices), chain(indices, [None]))
    return (alist[i:j] for i, j in pairs)

and of course, the very, very lazy guy solution (if you don't mind to get arrays instead of lists, but anyway you can always revert them to lists):

import numpy
partition = numpy.split

Answer 2

I would be interested in seeing a more Pythonic way of doing this also. But this is a crappy solution. You need to add a checking for an empty index list.

Something along the lines of:

indexes = [5, 12, 17]
list = range(20)

output = []
prev = 0

for index in indexes:
    output.append(list[prev:index])
    prev = index

output.append(list[indexes[-1]:])

print output

produces

[[0, 1, 2, 3, 4], [5, 6, 7, 8, 9, 10, 11], [12, 13, 14, 15, 16], [17, 18, 19]]