insert open and close tags at the positions corresponding to a list of tups

Question

I have to mark up a text inserting tags in a string as follows:

mystring = '123456789'
postions = [(2,4),(6,8)]

result = '12<tag1>34</tag1>56<tag2>78</tag2>9'

positions is a list of tuples that gives the start and end of the tag. (In principle the spans of the tags do not overlap)

I started assumend that I would need all the positions of the tags flatten out:

import itertools
pos2 = list(itertools.chain(*positions))

which gives:

[2, 4, 6, 8]

That allows me to do this:

stringWithTags= ''
opentag=True
nrtag=1
for j,character in enumerate(mystring):
    i=j+1
    if i in pos2:
        if opentag:
            toadd = character + '<tag' + str(nrtag) +'>' 
        else:
            toadd = character + '</tag' + str(nrtag) +'>'  
            nrtag = nrtag + 1
        
        stringWithTags = stringWithTags + toadd
        opentag = not(opentag)
        
    else:
        stringWithTags = stringWithTags + character

stringWithTags

that works, but its quite horrible code, and incredible verbose. This problem should be well known and there might be out of the box solutions I am not aware of.

Any suggestions?

EDIT 1: I also considered my code not water tight because the moment the spans overlap this will not fly and a solution valid in both cases

Answer 1

I think that easy way is to split string to list and work with it by indexes from positions like this: split string, loop over positions-list, add prefix tag or suffix tag by position, finally join list to new string.

mystring = '123456789'
postions = [(2,4),(6,8)]

str_list = [c for c in mystring]
for r in postions:
    for _, c in enumerate(r):
        str_list[c] = ('</tag>{}' if _ else '<tag>{}').format(str_list[c])
new_string = ''.join(str_list)