A Python script to replace relative paths in src/href attributes in a HTML string with absolute path equivalents. Inspiration was provided by https://gist.github.com/sprintingdev/8843526. See additional notes below.

relative_to_absolute_urls.py

# In development of this script I had four goals:
# 1) Operate on a string fragment of HTML and assume it could be poorly formed (ie - might not parse well with lxml, etc.)
# 2) Ability to call the function with base_url since it would be used in the processing of multiple sites and HTML fragments 
#    from them
# 3) Desire to use urlparse.urljoin to insure that the resulting absolute url was well formed
# 4) Even if the HTML fragment contains absolute URLs I could pass it through this routine without issue

# Starting with https://gist.github.com/sprintingdev/8843526 I found that certain 
# relative URLs would, when made absolute, were not always well formed.  Often there 
# could be multiple slashes in the URL path.  To combat this urlparse.urljoin was introduced.
#
# Additionally, due to my needs, I needed the flexibilty to pass base_url into the function called by 
# re.sub function.  To do this I used partials.  

import re
from functools import partial
from urlparse import urljoin

def srcrepl(base_url, match):
    absolute_link = urljoin(base_url, match.group(3))
    return "<" + match.group(1) + match.group(2) + "=" + "\"" + absolute_link + "\"" + match.group(4) + ">"

  
def relative_to_absolute_urls(fragment, base_url):
    p = re.compile(r"<(.*?)(src|href)=\"(?!http)(.*?)\"(.*?)>")
    absolute_fragment = p.sub(partial(srcrepl, base_url), fragment)

    return absolute_fragment