Question

I am writing a simple script to download .mp4 TEDTalks given a list of TEDTalk website links:

# Run through a list of TEDTalk website links and download each
# TEDTalk in high quality MP4

import urllib.request

#List of website links
l = [
        "http://www.ted.com/index.php/talks/view/id/28",
        "http://www.ted.com/index.php/talks/view/id/29",
    ]

# Function which takes the location of the string "-480p.mp4",
# d = 1 less that location, and a string and returns the
# full movie download link
def findFullURL(d, e, s):
    a = s[d]
    if a != "/":
        #Subtract from d to move back another letter
        d = d - 1
        findFullURL(d, e, s)
    else:
        fullURL = "http://download.ted.com/talks/" + s[(d+1):e] + "-480p.mp4"
        #print(fullURL)
        return fullURL

#Iterate through a list of links to download each movie
def iterateList(l):
    for x in l:
        #get the HTML
        f = urllib.request.urlopen(x)
        #Convert the HTML file into a string
        s = str(f.read(), "utf-8")
        f.close()
        #Find the location in the string where the interesting bit ends
        e = s.find("-480p.mp4")
        d = e - 1
        #The problem is with this variable url:
        url = findFullURL(d, e, s)
        print("Downloading " + url)
        #TODO: Download the file

I am certain that the function findFullURL works. If you uncomment the print(fullURL) line at the end of the findFullURL function, you will see it output the download link exactly as I need it.

However, in the iterateList function where I try to capture that string via url = findFullURL(d, e, s), the variable url seems to take on the value None. I do not understand this at all. It should be as simple as the following example, which works when I try it in the interpreter:

def hello():
    return "Hello"
url = hello()
print(url)

Answer 1

I am certain that the function findFullURL works.

Being certain that a certain piece of code works is the best way to waste hours of debugging time looking in the wrong place.

In fact that function does not work. You are missing a return:

def findFullURL(d, e, s):
    a = s[d]
    if a != "/":
        #Subtract from d to move back another letter
        d = d - 1
        return findFullURL(d, e, s)   # <<<<<<< here
    else:
        fullURL = "http://download.ted.com/talks/" + s[(d+1):e] + "-480p.mp4"
        #print(fullURL)
        return fullURL

Also, you shouldn’t be using recursion to solve this task. You can use rfind instead.

def findFullURL(d, e, s):
    d = s.rfind('/', 0, d + 1)
    # You probably want to handle the condition where '/' is not found here.
    return "http://download.ted.com/talks/" + s[(d+1):e] + "-480p.mp4"