Python's `urllib2`: Why do I get error 403 when I `urlopen` a Wikipedia page?

Wikipedias stance is:

Data retrieval: Bots may not be used to retrieve bulk content for any use not directly related to an approved bot task. This includes dynamically loading pages from another website, which may result in the website being blacklisted and permanently denied access. If you would like to download bulk content or mirror a project, please do so by downloading or hosting your own copy of our database.

That is why Python is blocked. You're supposed to download data dumps.

Anyways, you can read pages like this in Python 2:

req = urllib2.Request(url, headers={'User-Agent' : "Magic Browser"}) 
con = urllib2.urlopen( req )
print con.read()

Or in Python 3:

import urllib
req = urllib.request.Request(url, headers={'User-Agent' : "Magic Browser"}) 
con = urllib.request.urlopen( req )
print(con.read())

To debug this, you'll need to trap that exception.

try:
    f = urllib2.urlopen('http://en.wikipedia.org/wiki/OpenCola_(drink)')
except urllib2.HTTPError, e:
    print e.fp.read()

When I print the resulting message, it includes the following

"English

Our servers are currently experiencing a technical problem. This is probably temporary and should be fixed soon. Please try again in a few minutes. "

Often times websites will filter access by checking if they are being accessed by a recognised user agent. Wikipedia is just treating your script as a bot and rejecting it. Try spoofing as a browser. The following link takes to you an article to show you how.

http://wolfprojects.altervista.org/changeua.php