Avoiding 503 errors with urllib2

I'm new to web scraping with python, so I don't know if I'm doing this right.

I'm using a script that calls BeautifulSoup to parse the URLs from the first 10 pages of a google search. Tested with stackoverflow.com, worked just fine out-of-the-box. I tested with another site a few times, trying to see if the script was really working with higher google page requests, then it 503'd on me. I switched to another URL to test and worked for a couple, low-page requests, then also 503'd. Now every URL I pass to it is 503'ing. Any suggestions?

import sys # Used to add the BeautifulSoup folder the import path
import urllib2 # Used to read the html document

if __name__ == "__main__":
### Import Beautiful Soup
### Here, I have the BeautifulSoup folder in the level of this Python script
### So I need to tell Python where to look.
sys.path.append("./BeautifulSoup")
from BeautifulSoup import BeautifulSoup

### Create opener with Google-friendly user agent
opener = urllib2.build_opener()
opener.addheaders = [('User-agent', 'Mozilla/5.0')]

### Open page & generate soup
### the "start" variable will be used to iterate through 10 pages.
for start in range(0,10):
    url = "http://www.google.com/search?q=site:stackoverflow.com&start=" + str(start*10)
    page = opener.open(url)
    soup = BeautifulSoup(page)

    ### Parse and find
    ### Looks like google contains URLs in <cite> tags.
    ### So for each cite tag on each page (10), print its contents (url)
开发者_运维问答    for cite in soup.findAll('cite'):
        print cite.text

Automated querying is not permitted by Google Terms of Service. See this article for information: Unusual traffic from your computer and also Google Terms of service

As Ettore said, scraping the search results is against our ToS. However check out the WebSearch api, specifically the bottom section of the documentation which should give you a hint about how to access the API from non-javascipt environments.