from urllib import request

urls = (
    'http://math.stackexchange.com/election/5?tab=primary',
    'http://serverfault.com/election/5?tab=primary',
    'http://stackoverflow.com/election/7?tab=primary',
)

for url in urls:
    print('fetching {} ...'.format(url))
    request.urlopen(url).read()

fetching http://math.stackexchange.com/election/5?tab=primary ...
fetching http://serverfault.com/election/5?tab=primary ...
fetching http://stackoverflow.com/election/7?tab=primary ...
Traceback (most recent call last):
  File "examples/t.py", line 11, in <module>
    request.urlopen(url).read()
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/urllib/request.py", line 161, in urlopen
    return opener.open(url, data, timeout)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/urllib/request.py", line 469, in open
    response = meth(req, response)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/urllib/request.py", line 579, in http_response
    'http', request, response, code, msg, hdrs)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/urllib/request.py", line 507, in error
    return self._call_chain(*args)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/urllib/request.py", line 441, in _call_chain
    result = func(*args)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/urllib/request.py", line 587, in http_error_default
    raise HTTPError(req.full_url, code, msg, hdrs, fp)
urllib.error.HTTPError: HTTP Error 403: Forbidden

from urllib import request

urls = (
    'http://math.stackexchange.com/election/5?tab=primary',
    'http://serverfault.com/election/5?tab=primary',
    'http://stackoverflow.com/election/7?tab=primary',
)

for url in urls:
    print('fetching {} ...'.format(url))
    try:
        request.urlopen(url).read()
    except:
        print('got an exception, changing user-agent to urllib3 default')
        req = request.Request(url)
        req.add_header('User-Agent', 'Python-urllib/3.4')
        try:
            request.urlopen(req)
        except:
            print('got another exception, changing user-agent to something else')
            req.add_header('User-Agent', 'not-Python-urllib/3.4')
            request.urlopen(req)

fetching http://math.stackexchange.com/election/5?tab=primary ...
success with url: http://math.stackexchange.com/election/5?tab=primary

fetching http://serverfault.com/election/5?tab=primary ...
success with url: http://serverfault.com/election/5?tab=primary

fetching http://stackoverflow.com/election/7?tab=primary ...
got an exception, changing user-agent to urllib default
got another exception, changing user-agent to something else
success with url: http://stackoverflow.com/election/7?tab=primary

import requests

urls = (
    'http://math.stackexchange.com/election/5?tab=primary',
    'http://serverfault.com/election/5?tab=primary',
    'http://stackoverflow.com/election/7?tab=primary',
)

for url in urls:
    print('fetching {} ...'.format(url))
    data = requests.get(url)

import requests

urls = (
    'http://math.stackexchange.com/election/5?tab=primary',
    'http://serverfault.com/election/5?tab=primary',
    'http://stackoverflow.com/election/7?tab=primary',
)
with requests.Session() as session:
    for url in urls:
        print('fetching {} ...'.format(url))
        data = session.get(url)