urllib2 ======== urllib2.urlopen raises URLError or HTTPError in the event of an error. HTTPError is a subclass of URLError, which is a subclass of IOError. This means you can trap for IOError if you want. If the request fails to reach a server then urlopen raises a URLError. This will usually be because there is no network connection (no route to the specified server), or the specified server doesn't exist. In this case, the exception raised will have a 'reason' attribute, which is a tuple containing an error code and a text error message. e.g. >>> req = urllib2.Request('http://www.pretend_server.org') >>> try: >>> urllib2.urlopen(req) >>> except IOError, e: >>> print e.reason >>> (7, 'getaddrinfo failed') If the request reaches a server, but the server is unable to fulfil the request, it returns an error code. The default handlers will hande some of these errors for you. For those it can't handle, urlopen in will raise an ``HTTPError``. Typical errors include '404' (page not found), '403' (request forbidden), and '401' (authentication required). See http://www.w3.org/hypertext/WWW/Protocols/HTTP/HTRESP.html for a useful reference on the http error codes. The HTTPError instance raised will have an integer 'code' attribute, which corresponds to the error sent by the server. There is a useful dictionary of response codes in ``HTTPBaseServer``, that shows all the defined response codes. Because the default handlers handle redirects (codes in the 300 range), and codes in the 100-299 range indicate success, you will only ever see error codes in the 400-599 range. An exception to this is theoretically code 206 - partial content - but you *ought* to only get this in response to a request for partial content. # Table mapping response codes to messages; entries have the # form {code: (shortmessage, longmessage)}. httpresponses = { 100: ('Continue', 'Request received, please continue'), 101: ('Switching Protocols', 'Switching to new protocol; obey Upgrade header'), 200: ('OK', 'Request fulfilled, document follows'), 201: ('Created', 'Document created, URL follows'), 202: ('Accepted', 'Request accepted, processing continues off-line'), 203: ('Non-Authoritative Information', 'Request fulfilled from cache'), 204: ('No response', 'Request fulfilled, nothing follows'), 205: ('Reset Content', 'Clear input form for further input.'), 206: ('Partial Content', 'Partial content follows.'), 300: ('Multiple Choices', 'Object has several resources -- see URI list'), 301: ('Moved Permanently', 'Object moved permanently -- see URI list'), 302: ('Found', 'Object moved temporarily -- see URI list'), 303: ('See Other', 'Object moved -- see Method and URL list'), 304: ('Not modified', 'Document has not changed since given time'), 305: ('Use Proxy', 'You must use proxy specified in Location to access this ' 'resource.'), 307: ('Temporary Redirect', 'Object moved temporarily -- see URI list'), 400: ('Bad request', 'Bad request syntax or unsupported method'), 401: ('Unauthorized', 'No permission -- see authorization schemes'), 402: ('Payment required', 'No payment -- see charging schemes'), 403: ('Forbidden', 'Request forbidden -- authorization will not help'), 404: ('Not Found', 'Nothing matches the given URI'), 405: ('Method Not Allowed', 'Specified method is invalid for this server.'), 406: ('Not Acceptable', 'URI not available in preferred format.'), 407: ('Proxy Authentication Required', 'You must authenticate with ' 'this proxy before proceeding.'), 408: ('Request Time-out', 'Request timed out; try again later.'), 409: ('Conflict', 'Request conflict.'), 410: ('Gone', 'URI no longer exists and has been permanently removed.'), 411: ('Length Required', 'Client must specify Content-Length.'), 412: ('Precondition Failed', 'Precondition in headers is false.'), 413: ('Request Entity Too Large', 'Entity is too large.'), 414: ('Request-URI Too Long', 'URI is too long.'), 415: ('Unsupported Media Type', 'Entity body in unsupported format.'), 416: ('Requested Range Not Satisfiable', 'Cannot satisfy request range.'), 417: ('Expectation Failed', 'Expect condition could not be satisfied.'), 500: ('Internal error', 'Server got itself in trouble'), 501: ('Not Implemented', 'Server does not support this operation'), 502: ('Bad Gateway', 'Invalid responses from another server/proxy.'), 503: ('Service temporarily overloaded', 'The server cannot process the request due to a high load'), 504: ('Gateway timeout', 'The gateway server did not receive a timely response'), 505: ('HTTP Version not supported', 'Cannot fulfill request.'), } When an error is raised the server responds by returning an http error code *and* an error page. You can use the HTTPError instance as a handle on the page returned. This means that as well as the code attribute, it also has read, geturl, and info, methods. >>> req = urllib2.Request('http://www.python.org/fish.html') >>> try: urllib2.urlopen(req) >>> except IOError, e: >>> print e.code >>> print e.read() >>> 404 Error 404: File Not Found ...... etc... The handle returned by ``urlopen`` has two useful methods ``info`` and ``geturl``. * geturl - this returns the real url of the page fetched. This is useful because ``urlopen`` (or the openener object used) may have followed a redirect. The url of the page fetched may not be the same as the url requested. * info - this returns a dictionary like object that describes the page fetched, particularly the headers sent by the server. It is actually an ``httplib.HTTPMessage`` instance. In versions of Python prior to 2.3.4 it wasn't safe to iterate over the object directly, but you should iterate over the list returned by ``msg.keys()`` instead. Typical headers include 'content-length', 'content-type', and so on. See http://www.cs.tut.fi/~jkorpela/http.html for a useful reference on headers that can be sent by a server. ``build_opener`` can be used to create ``opener``objects - for fetching URLs with specific handlers installed. These can be used to handle cookies, authentication, and other common but slightly specialised situations. Opener objects have an ``open`` method, which can be called directly to fetch urls in the same way as the ``urlopen`` function. ``install_opener``can be used to make an ``opener``object the default opener. This means that calls to ``urlopen``will use the opener you have installed. To illustrate creating and installing a handler we will use the ``HTTPBasicAuthHandler``. When an error 401 is raised, the server sends a header (along with the error code) requesting authentication and specifying an authentication scheme and a 'realm'. The header looks like : www-authenticate: SCHEME realm="REALM" e.g. www-authenticate: Basic realm="cPanel" The client should then retry the request with the appropriate name and password for the realm included as a header in the request. This is 'basic authentication'. In order to simplify this process we can create an instance of ``HTTPBasicAuthHandler`` and an opener to use this handler. (See http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/305288 recipe for details of how Basic Authenitcation works, and how you could handle it without using a handler). The ``HTTPBasicAuthHandler`` uses an object called a password manager to handle the mapping of URIs and realms to passwords and usernames. If you know what the realm is (from the authentication header sent by the server), then you can use a ``HTTPPasswordMgr``. Generally there is only one realm per URI, so it is possible to use ``HTTPPasswordMgrWithDefaultRealm``. This allows you to specify a default username and password for a URI. This will be supplied in the absence of yoou providing an alternative combination for a specific realm. We signify this by providing ``None`` as the realm argument to the ``add_password`` method. The toplevelurl is the first url that requires authentication. This is usually a 'super-url' of any others in the same realm. password_mgr = urllib2.HTTPPasswordMgrWithDefaultRealm() # create a password manager password_mgr.add_password(None, top_level_url, username, password) # add the username and password handler = urllib2.HTTPBasicAuthHandler(password_mgr) # create the handler opener = urllib2.build_opener(handler) # from handler to opener opener.open(a_url) # use the opener to fetch a URL urllib2.install_opener(opener) # install the opener # now all calls to urllib2.urlopen use our opener You will note that in the above example we only supplied our ``HHTPBasicAuthHandler`` to ``build_opener``. By default every opener will include the handlers for the normal situations - ProxyHandler, UnknownHandler, HTTPHandler, HTTPDefaultErrorHandler, HTTPRedirectHandler, FTPHandler, FileHandler, HTTPErrorProcessor. The only reason to explicitly supply these to ``build_opener`` (which actually chains handlers provided as a list), would be to change the order they appear in the chain.