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print(fullURL)函数末尾的findFullURL行，您将看到它完全按照我的需要输出下载链接。

但是，在我尝试通过iterateList捕获该字符串的url = findFullURL(d, e, s)函数中，变量url似乎采用值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它时，它可以工作：

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

Answer 1

  

我确定函数findFullURL有效。

确定某段代码的工作原理是浪费时间调试时间错误的最佳方法。

事实上，这个功能不起作用。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。您可以改为使用rfind。

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"

Answer 2

findFullURL在return语句的第一个分支上没有if语句。在Python中，这意味着它返回None。