Python: how to get the next sequence of a list of lists based on a condition?

I used an NLP chunker that splits incorrectly the term ‘C++’ and ‘C#’ as: C (NN), +(SYM), +(SYM), C (NN), #(SYM).

The resulting list of incorrect chunking looks like this:

l = [['C', 'NN'], ['+', 'SYM'], ['+', 'SYM'], ['C', 'NN'], ['#', 'NN']]

I would like to post-process this list, by identifying the strings in index 0 of each list that are ‘C’ and the next in line ‘+’, ‘+’ or ‘#’. Then I’d like to concatenate these strings, so that ‘C’,’+’,’+’ becomes ‘C++’ by simply adding these together. This has to be generalisable, so it should work with lists that contain multiple different words, but still concatenate the desired strings.

desired result:

l_desired = [['C++', 'NN'], ['C#', 'NN']]

I can identify the items in the list independently (index 0) but I don’t know how to go about identifying the desired sequence. My idea was to use the next() function, although I do not know where to begin.

>Solution :

You can loop over the list and check if the first element is a letter, in this case append as a new item, else update the last item:

from string import ascii_letters

letters = set(ascii_letters)

out = []
for e in l:
    if e[0][0] in letters:
        out.append(e.copy()) # making a copy not to affect original list
    elif out: # this is to check that out is not empty (edge case)
        out[-1][0] += e[0]

Or using a blacklist of symbols:

symbols = set('+#')

out = []
for e in l:
    if e[0] in symbols and out:
        out[-1][0] += e[0]
    else:
        out.append(e.copy())

output:

[['C++', 'NN'], ['C#', 'NN']]