在python中迭代列表

Question

我在循环中有一个循环，我正在使用 try n catch 一旦出现错误 try n catch 工作正常，但循环继续到下一个值。我需要的是循环中断从相同的值开始的地方不要继续到下一个，所以我如何用我的代码做到这一点[就像在其他语言中一样：在 c++ 中，它是 i--]

for 
r = urllib2.urlopen(url) 
       encoding = r.info().getparam('charset')
       html = r.read()
c = td.find('a')['href']
urls = []
urls.append(c)
#collecting urls from first page then from those url collecting further info in below loop

    for abc in urls:
       try:
           r = urllib2.urlopen(abc) 
           encoding = r.info().getparam('charset')
           html = r.read()
       except Exception as e:
           last_error = e
           time.sleep(retry_timeout) #here is the problem once get error then switch from next value 

我需要一种更 Pythonic 的方式来做到这一点。 等待答复。谢谢。

Answer 1

不幸的是，没有简单的方法可以在 Python 中使用迭代器返回：

http://docs.python.org/2/library/stdtypes.html

你应该对这个 stackoverflow 的线程感兴趣： Making a python iterator go backwards?

对于您的特殊情况，我将使用一个简单的 while 循环：

url = []
i = 0
while i < len(url): #url is list contain all urls which contain infinite as url updates every day
    data = url[i]
    try:
        #getting data from there
        i+=1
    except:
        #shows the error received and continue to next loop i need to make the loop start from same position

您想要处理问题的方式的问题是您将冒着陷入无限循环的风险。例如，如果链接损坏 r = urllib2.urlopen(abc) 将始终运行异常并且您将始终停留在同一位置。你应该考虑做这样的事情：

r = urllib2.urlopen(url) 
encoding = r.info().getparam('charset')
html = r.read()
c = td.find('a')['href']
urls = []
urls.append(c)
#collecting urls from first page then from those url collecting further info in below loop
NUM_TRY = 3
for abc in urls:
   for _ in range(NUM_TRY):
       try:
           r = urllib2.urlopen(abc) 
           encoding = r.info().getparam('charset')
           html = r.read()
           break #if we arrive to this line, it means no error occur so we don't need to retry again
                #this is why we break the inner loop
       except Exception as e:
           last_error = e
           time.sleep(retry_timeout) #here is the problem once get error then switch from next value