Takes a KML with multiple LineString's and merges them into one, sequentially.

kml_mergelines.py

#!/usr/bin/env python

import codecs
from bs4 import BeautifulSoup
import sys

contents = open(sys.argv[1]).read()

soup = BeautifulSoup(contents, features="xml")
allLineStrings = soup.find_all("LineString")

singleLine = u""

for ls in allLineStrings:
    singleLine += " "+" ".join(list(ls.stripped_strings))

for ls in allLineStrings:
    ls.parent.decompose()

newTag = BeautifulSoup("<Placemark><name>Percurso</name><LineString><coordinates/></LineString></Placemark>",features="xml")
newTag.find("coordinates").string = singleLine

soup.find("Document").append(newTag.contents[0])

codecs.open(sys.argv[1].replace(".kml","_merged.kml"), mode='w', encoding="utf-8").write(soup.prettify())