Doubly encoded URLs in coursera.

funny.py

import urllib

# Some silly people put invalid URLs like this:
#
#    'https://d396qusza40orc.cloudfront.net/webapplications/https%3A//d396qusza40orc.cloudfront.net/webapplications/lecture_slides/M6-L8-Ajax-Handout.pdf'
#
# as resources for students to download. We work around incompetence
# because, well, we are not like them.

s = 'https://d396qusza40orc.cloudfront.net/webapplications/https%3A//d396qusza40orc.cloudfront.net/webapplications/lecture_slides/M6-L8-Ajax-Handout.pdf'

s_unquoted = urllib.unquote(s)
pos = s_unquoted.rfind('https://')

suffix = s_unquoted[pos:]
prefix = s_unquoted[:pos]

url = suffix if suffix.startswith(prefix) else s_unquoted

print(url)