Python执行一个“ for”循环，但不执行第二个

Question

My python code executes the first 'for' loop, but not the second. If I comment out the first for loop, the second executes properly.

Code:

import urllib.request
from bs4 import BeautifulSoup 
from bs4 import NavigableString

site = urllib.request.urlopen('http://www.reddit.com/')
html = site.read()

soup = BeautifulSoup(html)
tags = soup.body.children

for item in tags:        
    print (item.name)
    print (item.attrs)

for item in tags:
    if ('role' in item.attrs and item.attrs['role'] == 'banner'):
        print (item)
    else: pass

any ideas? My IDLE gui is acting strange tonight (shift+indent is indenting instead of undenting for example) so if this works for everyone else properly it might just be my box.

Thanks community!

Answer 1

I've never used BeautifulSoup, but it sounds a lot like tags is an iterator, not a list or something similar. That means, iter(tags) is tags and it has state that is altered by next() . As a consequence, iterating over it consumes it irreparably. I'd also wager that body.children is a property that creates a new iterator on each access, so you can run both loops if you turn it into a list first, or repeat tags = soup.body.children after the first loop.

Answer 2

tags is an iterable, but not a sequence; the first for loop exhausts the iterable, leaving no elements for the second. Pass it to the list or tuple constructor to create a sequence from it, then iterate over that.