Python 3.2.2函数在非空变量上返回None

Question

我正在编写一个简单的脚本来下载.mp4 TEDTalks给出一个TEDTalk网站链接列表:

# 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

我确信函数findFullURL有效.如果取消注释函数print(fullURL)末尾的行findFullURL,您将看到它完全按照我的需要输出下载链接.

但是,在iterateList我尝试捕获该字符串的函数中url = findFullURL(d, e, s),变量url似乎采用了该值None.我根本不明白这一点.它应该像下面的例子一样简单,当我在解释器中尝试时它可以工作:

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

Answer 1

我确信函数findFullURL有效.

确保某段代码可以正常运行,这是浪费数小时调试时间在错误位置的最佳方法.

事实上,该功能不起作用.你错过了一个回报:

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

此外,您不应该使用递归来解决此任务.你可以rfind改用.

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"