python string split by separator all possible permutations

Question

This might be heavily related to similar questions as Python 3.3: Split string and create all combinations , but I can't infer a pythonic solution out of this.

Question is:

Let there be a str such as 'hi|guys|whats|app', and I need all permutations of splitting that str by a separator. Example:

#splitting only once
['hi','guys|whats|app']
['hi|guys','whats|app']
['hi|guys|whats','app']
#splitting only twice
['hi','guys','whats|app']
['hi','guys|whats','app']
#splitting only three times
...
etc

I could write a backtracking algorithm, but does python (itertools, e.g.) offer a library that simplifies this algorithm?

Thanks in advance!!

Answer 1

An approach, once you have split the string is to use itertools.combinations to define the split points in the list, the other positions should be fused again.

def lst_merge(lst, positions, sep='|'):
    '''merges a list on points other than positions'''
    '''A, B, C, D and 0, 1 -> A, B, C|D'''
    a = -1
    out = []
    for b in list(positions)+[len(lst)-1]:
        out.append('|'.join(lst[a+1:b+1]))
        a = b
    return out

def split_comb(s, split=1, sep='|'):
    from itertools import combinations
    l = s.split(sep)
    return [lst_merge(l, pos, sep=sep)
            for pos in combinations(range(len(l)-1), split)]

examples

>>> split_comb('hi|guys|whats|app', 0)
[['hi|guys|whats|app']]

>>> split_comb('hi|guys|whats|app', 1)
[['hi', 'guys|whats|app'],
 ['hi|guys', 'whats|app'],
 ['hi|guys|whats', 'app']]

>>> split_comb('hi|guys|whats|app', 2)
[['hi', 'guys', 'whats|app'],
 ['hi', 'guys|whats', 'app'],
 ['hi|guys', 'whats', 'app']]

>>> split_comb('hi|guys|whats|app', 3)
[['hi', 'guys', 'whats', 'app']]

>>> split_comb('hi|guys|whats|app', 4)
[] ## impossible

rationale

ABCD -> A B C D
         0 1 2

combinations of split points: 0/1 or 0/2 or 1/2

0/1 -> merge on 2 -> A B CD
0/2 -> merge on 1 -> A BC D
1/2 -> merge on 0 -> AB C D

generic function

Here is a generic version, working like above but also taking -1 as parameter for split, in which case it will output all combinations

def lst_merge(lst, positions, sep='|'):
    a = -1
    out = []
    for b in list(positions)+[len(lst)-1]:
        out.append('|'.join(lst[a+1:b+1]))
        a = b
    return out

def split_comb(s, split=1, sep='|'):
    from itertools import combinations, chain
    
    l = s.split(sep)
    
    if split == -1:
        pos = chain.from_iterable(combinations(range(len(l)-1), r)
                                  for r in range(len(l)+1))
    else:
        pos = combinations(range(len(l)-1), split)
        
    return [lst_merge(l, pos, sep=sep)
            for pos in pos]

example:

>>> split_comb('hi|guys|whats|app', -1)
[['hi|guys|whats|app'],
 ['hi', 'guys|whats|app'],
 ['hi|guys', 'whats|app'],
 ['hi|guys|whats', 'app'],
 ['hi', 'guys', 'whats|app'],
 ['hi', 'guys|whats', 'app'],
 ['hi|guys', 'whats', 'app'],
 ['hi', 'guys', 'whats', 'app']]