Split a list into nested lists on a value

Question

Say I have a list like so:

[1, 4, None, 6, 9, None, 3, 9, 4 ]

I decide to split this into nested lists on None, to get this:

[ [ 1, 4 ], [ 6, 9 ], [ 3, 9, 4 ] ]

Of course, I could have wanted to do this on (9, None) in which case, we would have got:

[ [ 1, 4 ], [ 6 ], [ 3 ], [ 4 ] ]

This is trivial to do using list append through iteration ( in a for loop )

I am interested to know whether this can be done in something faster - like a list comprehension?

If not, why not ( for example, a list comprehension cannot return more than one list element per iteration? )

Answer 1

Something like this:

def _itersplit(l, splitters):
    current = []
    for item in l:
        if item in splitters:
            yield current
            current = []
        else:
            current.append(item)
    yield current

def magicsplit(l, *splitters):
    return [subl for subl in _itersplit(l, splitters) if subl]

gets me:

>>> l = [1, 4, None, 6, 9, None, 3, 9, 4 ]
>>> magicsplit(l, None)
[[1, 4], [6, 9], [3, 9, 4]]
>>> magicsplit(l, None, 9)
[[1, 4], [6], [3], [4]]

Answer 2

The problem with trying to use a list comprehension for this is that the comprehension is inherently stateless, and you need state to perform the splitting operation. In particular, you need to remember, from one element to the next, what elements were found after the previous 'split' marker.

However, you could use one list comprehension to extract the indices of the split elements, and then another to use those indices to chop up the list. We need to translate the split indices into (begin, end) indices for the necessary 'pieces'. What we'll do is transform the list of split indices into two separate lists of 'begins' and 'ends', and then zip them together.

The whole thing looks like:

splitters = (9, None) # for example
indices = [i for (i, x) in enumerate(original) if x in splitters]
ends = indices + [len(original)]
begins = [0] + [x + 1 for x in indices]
result = [original[begin:end] for (begin, end) in zip(begins, ends)]

Answer 3

You could find the indices of the "delimiter" elements. For example, the indices of None are 2 and 5 (zero-based). You could use these, along with the length of the list, to construct a list of tuples [ (0,1), (3,4), (6,8) ] representing the start indices and end indices of the sublists. Then you can use a list comprehension over this list of tuples to extract sublists.

Answer 4

Here's an implementation using reduce, with a few bells and whistles:

#!/usr/bin/env python

def split_on (delims, seq, remove_empty=True):
    '''Split seq into lists using delims as a delimiting elements.

    For example, split_on(delims=2, list=xrange(0,5)) yields [ [0,1], [3,4] ].

    delims can be either a single delimiting element or a list or
    tuple of multiple delimiting elements. If you wish to use a list
    or tuple as a delimiter, you must enclose it in another list or
    tuple.

    If remove_empty is False, then consecutive delimiter elements or delimiter elements at the beginning or end of the longlist'''
    if type(delims) not in (type(list()), type(tuple())):
        delims = ( delims, )
    def reduce_fun(lists, elem):
        if elem in delims:
            if remove_empty and lists[-1] == []:
                # Avoid adding multiple empty lists
                pass
            else:
                lists.append([])
        else:
            lists[-1].append(elem)
        return lists
    result_list = reduce(reduce_fun, seq, [ [], ])
    # Maybe remove trailing empty list
    if remove_empty and result_list[-1] == []:
        result_list.pop()
    return result_list

mylist = [1, 4, None, 6, 9, None, 3, 9, 4 ]

print(split_on(None, mylist))
print(split_on((9, None), mylist))

Answer 5

>>> def isplit(iterable,splitters):
    return [list(g) for k,g in itertools.groupby(iterable,lambda x:x in splitters) if not k]

>>> isplit(L,(None,))
[[1, 4], [6, 9], [3, 9, 4]]
>>> isplit(L,(None,9))
[[1, 4], [6], [3], [4]]

benchmark code:

import timeit    

kabie=("isplit_kabie",
"""
import itertools
def isplit_kabie(iterable,splitters):
    return [list(g) for k,g in itertools.groupby(iterable,lambda x:x in splitters) if not k]
""" )

ssplit=("ssplit",
"""
def ssplit(seq,splitters):
    seq=list(seq)
    if splitters and seq:
        result=[]
        begin=0
        for end in range(len(seq)):
            if seq[end] in splitters:
                if end > begin:
                    result.append(seq[begin:end])
                begin=end+1
        if begin<len(seq):
            result.append(seq[begin:])
        return result
    return [seq]
""" )

ssplit2=("ssplit2",
"""
def ssplit2(seq,splitters):
    seq=list(seq)
    if splitters and seq:
        splitters=set(splitters).intersection(seq)
        if splitters:
            result=[]
            begin=0
            for end in range(len(seq)):
                if seq[end] in splitters:
                    if end > begin:
                        result.append(seq[begin:end])
                    begin=end+1
            if begin<len(seq):
                result.append(seq[begin:])
            return result
    return [seq]
""" )

emile=("magicsplit",
"""
def _itersplit(l, *splitters):
    current = []
    for item in l:
        if item in splitters:
            yield current
            current = []
        else:
            current.append(item)
    yield current

def magicsplit(l, splitters):
    return [subl for subl in _itersplit(l, *splitters) if subl]
""" )

emile_improved=("magicsplit2",
"""
def _itersplit(l, *splitters):
    current = []
    for item in l:
        if item in splitters:
            if current:
                yield current
                current = []
        else:
            current.append(item)
    if current:
        yield current

def magicsplit2(l, splitters):
    if splitters and l:
        return [i for i in _itersplit(l, *splitters)]
    return [list(l)]
""" )

karl=("ssplit_karl",
"""
def ssplit_karl(original,splitters):
    indices = [i for (i, x) in enumerate(original) if x in splitters]
    ends = indices + [len(original)]
    begins = [0] + [x + 1 for x in indices]
    return [original[begin:end] for (begin, end) in zip(begins, ends)]
""" )

ryan=("split_on",
"""
from functools import reduce
def split_on (seq, delims, remove_empty=True):
    '''Split seq into lists using delims as a delimiting elements.

    For example, split_on(delims=2, list=xrange(0,5)) yields [ [0,1], [3,4] ].

    delims can be either a single delimiting element or a list or
    tuple of multiple delimiting elements. If you wish to use a list
    or tuple as a delimiter, you must enclose it in another list or
    tuple.

    If remove_empty is False, then consecutive delimiter elements or delimiter elements at the beginning or end of the longlist'''
    delims=set(delims)
    def reduce_fun(lists, elem):
        if elem in delims:
            if remove_empty and lists[-1] == []:
                # Avoid adding multiple empty lists
                pass
            else:
                lists.append([])
        else:
            lists[-1].append(elem)
        return lists
    result_list = reduce(reduce_fun, seq, [ [], ])
    # Maybe remove trailing empty list
    if remove_empty and result_list[-1] == []:
        result_list.pop()
    return result_list
""" )

cases=(kabie, emile, emile_improved, ssplit ,ssplit2 ,ryan)

data=(
    ([1, 4, None, 6, 9, None, 3, 9, 4 ],(None,)),
    ([1, 4, None, 6, 9, None, 3, 9, 4 ]*5,{None,9,7}),
    ((),()),
    (range(1000),()),
    ("Split me",('','')),
    ("split me "*100,' '),
    ("split me,"*100,' ,'*20),
    ("split me, please!"*100,' ,!'),
    (range(100),range(100)),
    (range(100),range(101,1000)),
    (range(100),range(50,150)),
    (list(range(100))*30,(99,)),
    )

params="seq,splitters"

def benchmark(func,code,data,params='',times=10000,rounds=3,debug=''):
    assert(func.isidentifier())
    tester = timeit.Timer(stmt='{func}({params})'.format(
                                func=func,params=params),
                          setup="{code}\n".format(code=code)+
            (params and "{params}={data}\n".format(params=params,data=data)) +
            (debug and """ret=repr({func}({params}))
print({func}.__name__.rjust(16),":",ret[:30]+"..."+ret[-15:] if len(ret)>50 else ret)
                       """.format(func=func,params=params)))
    results = [tester.timeit(times) for i in range(rounds)]
    if not debug:
        print("{:>16s} takes:{:6.4f},avg:{:.2e},best:{:.4f},worst:{:.4f}".format(
            func,sum(results),sum(results)/times/rounds,min(results),max(results)))

def testAll(cases,data,params='',times=10000,rounds=3,debug=''):
    if debug:
        times,rounds = 1,1
    for dat in data:
        sdat = tuple(map(repr,dat))
        print("{}x{} times:".format(times,rounds),
              ','.join("{}".format(d[:8]+"..."+d[-5:] if len(d)>16 else d)for d in map(repr,dat)))
        for func,code in cases:
            benchmark(func,code,dat,params,times,rounds,debug)

if __name__=='__main__':
    testAll(cases,data,params,500,10)#,debug=True)

Output on i3-530, Windows7, Python 3.1.2:

500x10 times: [1, 4, N...9, 4],(None,)
    isplit_kabie takes:0.0605,avg:1.21e-05,best:0.0032,worst:0.0074
      magicsplit takes:0.0287,avg:5.74e-06,best:0.0016,worst:0.0036
     magicsplit2 takes:0.0174,avg:3.49e-06,best:0.0017,worst:0.0018
          ssplit takes:0.0149,avg:2.99e-06,best:0.0015,worst:0.0016
         ssplit2 takes:0.0198,avg:3.96e-06,best:0.0019,worst:0.0021
        split_on takes:0.0229,avg:4.59e-06,best:0.0023,worst:0.0024
500x10 times: [1, 4, N...9, 4],{9, None, 7}
    isplit_kabie takes:0.1448,avg:2.90e-05,best:0.0144,worst:0.0146
      magicsplit takes:0.0636,avg:1.27e-05,best:0.0063,worst:0.0065
     magicsplit2 takes:0.0891,avg:1.78e-05,best:0.0064,worst:0.0162
          ssplit takes:0.0593,avg:1.19e-05,best:0.0058,worst:0.0061
         ssplit2 takes:0.1004,avg:2.01e-05,best:0.0069,worst:0.0142
        split_on takes:0.0929,avg:1.86e-05,best:0.0090,worst:0.0096
500x10 times: (),()
    isplit_kabie takes:0.0041,avg:8.14e-07,best:0.0004,worst:0.0004
      magicsplit takes:0.0040,avg:8.04e-07,best:0.0004,worst:0.0004
     magicsplit2 takes:0.0022,avg:4.35e-07,best:0.0002,worst:0.0002
          ssplit takes:0.0023,avg:4.59e-07,best:0.0002,worst:0.0003
         ssplit2 takes:0.0023,avg:4.53e-07,best:0.0002,worst:0.0002
        split_on takes:0.0072,avg:1.45e-06,best:0.0007,worst:0.0009
500x10 times: range(0, 1000),()
    isplit_kabie takes:0.8892,avg:1.78e-04,best:0.0881,worst:0.0895
      magicsplit takes:0.6614,avg:1.32e-04,best:0.0654,worst:0.0673
     magicsplit2 takes:0.0958,avg:1.92e-05,best:0.0094,worst:0.0099
          ssplit takes:0.0943,avg:1.89e-05,best:0.0093,worst:0.0095
         ssplit2 takes:0.0943,avg:1.89e-05,best:0.0093,worst:0.0096
        split_on takes:1.3348,avg:2.67e-04,best:0.1328,worst:0.1340
500x10 times: 'Split me',('', '')
    isplit_kabie takes:0.0234,avg:4.68e-06,best:0.0023,worst:0.0024
      magicsplit takes:0.0126,avg:2.52e-06,best:0.0012,worst:0.0013
     magicsplit2 takes:0.0138,avg:2.76e-06,best:0.0013,worst:0.0015
          ssplit takes:0.0119,avg:2.39e-06,best:0.0012,worst:0.0012
         ssplit2 takes:0.0075,avg:1.50e-06,best:0.0007,worst:0.0008
        split_on takes:0.0191,avg:3.83e-06,best:0.0018,worst:0.0023
500x10 times: 'split m... me ',' '
    isplit_kabie takes:2.0803,avg:4.16e-04,best:0.2060,worst:0.2098
      magicsplit takes:0.9219,avg:1.84e-04,best:0.0920,worst:0.0925
     magicsplit2 takes:1.0221,avg:2.04e-04,best:0.1018,worst:0.1034
          ssplit takes:0.8294,avg:1.66e-04,best:0.0818,worst:0.0834
         ssplit2 takes:0.9911,avg:1.98e-04,best:0.0983,worst:0.1014
        split_on takes:1.5672,avg:3.13e-04,best:0.1543,worst:0.1694
500x10 times: 'split m... me,',' , , , ... , ,'
    isplit_kabie takes:2.1847,avg:4.37e-04,best:0.2164,worst:0.2275
      magicsplit takes:3.7135,avg:7.43e-04,best:0.3693,worst:0.3783
     magicsplit2 takes:3.8104,avg:7.62e-04,best:0.3795,worst:0.3884
          ssplit takes:0.9522,avg:1.90e-04,best:0.0939,worst:0.0956
         ssplit2 takes:1.0140,avg:2.03e-04,best:0.1009,worst:0.1023
        split_on takes:1.5747,avg:3.15e-04,best:0.1563,worst:0.1615
500x10 times: 'split m...ase!',' ,!'
    isplit_kabie takes:3.3443,avg:6.69e-04,best:0.3324,worst:0.3380
      magicsplit takes:2.0594,avg:4.12e-04,best:0.2054,worst:0.2076
     magicsplit2 takes:2.1850,avg:4.37e-04,best:0.2180,worst:0.2191
          ssplit takes:1.4881,avg:2.98e-04,best:0.1484,worst:0.1493
         ssplit2 takes:1.8779,avg:3.76e-04,best:0.1868,worst:0.1920
        split_on takes:2.9596,avg:5.92e-04,best:0.2946,worst:0.2980
500x10 times: range(0, 100),range(0, 100)
    isplit_kabie takes:0.9445,avg:1.89e-04,best:0.0933,worst:0.1023
      magicsplit takes:0.5878,avg:1.18e-04,best:0.0583,worst:0.0593
     magicsplit2 takes:0.5597,avg:1.12e-04,best:0.0554,worst:0.0588
          ssplit takes:0.8568,avg:1.71e-04,best:0.0852,worst:0.0874
         ssplit2 takes:0.1399,avg:2.80e-05,best:0.0121,worst:0.0242
        split_on takes:0.1462,avg:2.92e-05,best:0.0145,worst:0.0148
500x10 times: range(0, 100),range(101, 1000)
    isplit_kabie takes:19.9749,avg:3.99e-03,best:1.9789,worst:2.0330
      magicsplit takes:9.4997,avg:1.90e-03,best:0.9369,worst:0.9640
     magicsplit2 takes:9.4394,avg:1.89e-03,best:0.9267,worst:0.9665
          ssplit takes:19.2363,avg:3.85e-03,best:1.8936,worst:1.9516
         ssplit2 takes:0.2032,avg:4.06e-05,best:0.0201,worst:0.0205
        split_on takes:0.3329,avg:6.66e-05,best:0.0323,worst:0.0344
500x10 times: range(0, 100),range(50, 150)
    isplit_kabie takes:1.1394,avg:2.28e-04,best:0.1130,worst:0.1153
      magicsplit takes:0.7288,avg:1.46e-04,best:0.0721,worst:0.0760
     magicsplit2 takes:0.7220,avg:1.44e-04,best:0.0705,worst:0.0774
          ssplit takes:1.0835,avg:2.17e-04,best:0.1059,worst:0.1116
         ssplit2 takes:0.1092,avg:2.18e-05,best:0.0105,worst:0.0116
        split_on takes:0.1639,avg:3.28e-05,best:0.0162,worst:0.0168
500x10 times: [0, 1, 2..., 99],(99,)
    isplit_kabie takes:3.2579,avg:6.52e-04,best:0.3225,worst:0.3360
      magicsplit takes:2.2937,avg:4.59e-04,best:0.2274,worst:0.2344
     magicsplit2 takes:2.6054,avg:5.21e-04,best:0.2587,worst:0.2642
          ssplit takes:1.5251,avg:3.05e-04,best:0.1495,worst:0.1729
         ssplit2 takes:1.7298,avg:3.46e-04,best:0.1696,worst:0.1858
        split_on takes:4.1041,avg:8.21e-04,best:0.4033,worst:0.4291

Slightly modified Ryan's code, hope you don't mind. ssplit was based on the idea of Karl. Added statements handling some special cases to became ssplit2 which is the best solution I may provide.