import pywikibot
import re
from pywikibot import pagegenerators
site = pywikibot.Site()
page = pywikibot.Page(site, u"User:PearBOT/test")
text = page.text
templates=pywikibot.textlib.extract_templates_and_params(text)
def extractParameterValue(parmeter_name):
    regex = parameter_name + "="
    for x in templates:
        if re.search(x):
            return re.replace(regex,,x)
text += extractParameterValue(length main)
page.text = text
page.save(u"Bot test")