Most Pythonic Way to Split an Array by Repeating Elements

Question

I have a list of items that I want to split based on a delimiter. I want all delimiters to be removed and the list to be split when a delimiter occurs twice. For example, if the delimiter is 'X', then the following list:

['a', 'b', 'X', 'X', 'c', 'd', 'X', 'X', 'f', 'X', 'g']

Would turn into:

[['a', 'b'], ['c', 'd'], ['f', 'g']]

Notice that the last set is not split.

I've written some ugly code that does this, but I'm sure there is something nicer. Extra points if you can set an arbitrary length delimiter (i.e. split the list after seeing N delimiters).

Answer 1

I don't think there's going to be a nice, elegant solution to this (I'd love to be proven wrong of course) so I would suggest something straightforward:

def nSplit(lst, delim, count=2):
    output = [[]]
    delimCount = 0
    for item in lst:
        if item == delim:
            delimCount += 1
        elif delimCount >= count:
            output.append([item])
            delimCount = 0
        else:
            output[-1].append(item)
            delimCount = 0
    return output

 

>>> nSplit(['a', 'b', 'X', 'X', 'c', 'd', 'X', 'X', 'f', 'X', 'g'], 'X', 2)
[['a', 'b'], ['c', 'd'], ['f', 'g']]

Answer 2

Here's a way to do it with itertools.groupby():

import itertools

class MultiDelimiterKeyCallable(object):
    def __init__(self, delimiter, num_wanted=1):
        self.delimiter = delimiter
        self.num_wanted = num_wanted

        self.num_found = 0

    def __call__(self, value):
        if value == self.delimiter:
            self.num_found += 1
            if self.num_found >= self.num_wanted:
                self.num_found = 0
                return True
        else:
            self.num_found = 0

def split_multi_delimiter(items, delimiter, num_wanted):
    keyfunc = MultiDelimiterKeyCallable(delimiter, num_wanted)

    return (list(item
                 for item in group
                 if item != delimiter)
            for key, group in itertools.groupby(items, keyfunc)
            if not key)

items = ['a', 'b', 'X', 'X', 'c', 'd', 'X', 'X', 'f', 'X', 'g']

print list(split_multi_delimiter(items, "X", 2))

I must say that cobbal's solution is much simpler for the same results.