python urllib2

20.6. urllib2 — extensible library for opening URLs

Note

 

The urllib2 module has been split across several modules in Python 3 named urllib.request and urllib.error. The 2to3 tool will automatically adapt imports when converting your sources to Python 3.

The urllib2 module defines functions and classes which help in opening URLs (mostly HTTP) in a complex world — basic and digest authentication, redirections, cookies and more.

See also

 

The Requests package is recommended for a higher-level HTTP client interface.

The urllib2 module defines the following functions:

urllib2.urlopen(url[, data[, timeout[, cafile[, capath[, cadefault[, context]]]]])

Open the URL url, which can be either a string or a Request object.

data may be a string specifying additional data to send to the server, or None if no such data is needed. Currently HTTP requests are the only ones that use data; the HTTP request will be a POST instead of a GET when the data parameter is provided. data should be a buffer in the standard application/x-www-form-urlencoded format. The urllib.urlencode() function takes a mapping or sequence of 2-tuples and returns a string in this format. urllib2 module sends HTTP/1.1 requests with Connection:close header included.

The optional timeout parameter specifies a timeout in seconds for blocking operations like the connection attempt (if not specified, the global default timeout setting will be used). This actually only works for HTTP, HTTPS and FTP connections.

If context is specified, it must be a ssl.SSLContext instance describing the various SSL options. See HTTPSConnection for more details.

The optional cafile and capath parameters specify a set of trusted CA certificates for HTTPS requests. cafile should point to a single file containing a bundle of CA certificates, whereas capath should point to a directory of hashed certificate files. More information can be found in ssl.SSLContext.load_verify_locations().

The cadefault parameter is ignored.

This function returns a file-like object with three additional methods:

• geturl() — return the URL of the resource retrieved, commonly used to determine if a redirect was followed
• info() — return the meta-information of the page, such as headers, in the form of an mimetools.Message instance (see Quick Reference to HTTP Headers)
• getcode() — return the HTTP status code of the response.

Raises URLError on errors.

Note that None may be returned if no handler handles the request (though the default installed global OpenerDirector uses UnknownHandler to ensure this never happens).

In addition, if proxy settings are detected (for example, when a *_proxy environment variable like http_proxy is set), ProxyHandler is default installed and makes sure the requests are handled through the proxy.

Changed in version 2.6: timeout was added.

Changed in version 2.7.9: cafile, capath, cadefault, and context were added.

urllib2.install_opener(opener)

Install an OpenerDirector instance as the default global opener. Installing an opener is only necessary if you want urlopen to use that opener; otherwise, simply call OpenerDirector.open() instead of urlopen(). The code does not check for a real OpenerDirector, and any class with the appropriate interface will work.

urllib2.build_opener([handler, ...])

Return an OpenerDirector instance, which chains the handlers in the order given. handlers can be either instances of BaseHandler, or subclasses of BaseHandler (in which case it must be possible to call the constructor without any parameters). Instances of the following classes will be in front of the handlers, unless the handlers contain them, instances of them or subclasses of them: ProxyHandler (if proxy settings are detected), UnknownHandler, HTTPHandler, HTTPDefaultErrorHandler, HTTPRedirectHandler, FTPHandler, FileHandler, HTTPErrorProcessor.

If the Python installation has SSL support (i.e., if the ssl module can be imported), HTTPSHandler will also be added.

Beginning in Python 2.3, a BaseHandler subclass may also change its handler_order attribute to modify its position in the handlers list.

The following exceptions are raised as appropriate:

exception urllib2.URLError

The handlers raise this exception (or derived exceptions) when they run into a problem. It is a subclass of IOError.

reason

The reason for this error. It can be a message string or another exception instance (socket.error for remote URLs, OSError for local URLs).

exception urllib2.HTTPError

Though being an exception (a subclass of URLError), an HTTPError can also function as a non-exceptional file-like return value (the same thing that urlopen() returns). This is useful when handling exotic HTTP errors, such as requests for authentication.

code

An HTTP status code as defined in RFC 2616. This numeric value corresponds to a value found in the dictionary of codes as found in BaseHTTPServer.BaseHTTPRequestHandler.responses.

reason

The reason for this error. It can be a message string or another exception instance.

The following classes are provided:

class urllib2.Request(url[, data][, headers][, origin_req_host][, unverifiable])

This class is an abstraction of a URL request.

url should be a string containing a valid URL.

data may be a string specifying additional data to send to the server, or None if no such data is needed. Currently HTTP requests are the only ones that use data; the HTTP request will be a POST instead of a GET when the data parameter is provided. data should be a buffer in the standard application/x-www-form-urlencoded format. The urllib.urlencode() function takes a mapping or sequence of 2-tuples and returns a string in this format.

headers should be a dictionary, and will be treated as if add_header() was called with each key and value as arguments. This is often used to “spoof” the User-Agent header value, which is used by a browser to identify itself – some HTTP servers only allow requests coming from common browsers as opposed to scripts. For example, Mozilla Firefox may identify itself as "Mozilla/5.0 (X11; U;Linux i686) Gecko/20071127 Firefox/2.0.0.11", while urllib2’s default user agent string is "Python-urllib/2.6" (on Python 2.6).

The final two arguments are only of interest for correct handling of third-party HTTP cookies:

origin_req_host should be the request-host of the origin transaction, as defined by RFC 2965. It defaults to cookielib.request_host(self). This is the host name or IP address of the original request that was initiated by the user. For example, if the request is for an image in an HTML document, this should be the request-host of the request for the page containing the image.

unverifiable should indicate whether the request is unverifiable, as defined by RFC 2965. It defaults to False. An unverifiable request is one whose URL the user did not have the option to approve. For example, if the request is for an image in an HTML document, and the user had no option to approve the automatic fetching of the image, this should be true.

class urllib2.OpenerDirector

The OpenerDirector class opens URLs via BaseHandlers chained together. It manages the chaining of handlers, and recovery from errors.

class urllib2.BaseHandler

This is the base class for all registered handlers — and handles only the simple mechanics of registration.

class urllib2.HTTPDefaultErrorHandler

A class which defines a default handler for HTTP error responses; all responses are turned into HTTPError exceptions.

class urllib2.HTTPRedirectHandler

A class to handle redirections.

class urllib2.HTTPCookieProcessor([cookiejar])

A class to handle HTTP Cookies.

class urllib2.ProxyHandler([proxies])

Cause requests to go through a proxy. If proxies is given, it must be a dictionary mapping protocol names to URLs of proxies. The default is to read the list of proxies from the environment variables <protocol>_proxy. If no proxy environment variables are set, then in a Windows environment proxy settings are obtained from the registry’s Internet Settings section, and in a Mac OS X environment proxy information is retrieved from the OS X System Configuration Framework.

To disable autodetected proxy pass an empty dictionary.

Note

 

HTTP_PROXY will be ignored if a variable REQUEST_METHOD is set; see the documentation on getproxies().

class urllib2.HTTPPasswordMgr

Keep a database of (realm, uri) -> (user, password) mappings.

class urllib2.HTTPPasswordMgrWithDefaultRealm

Keep a database of (realm, uri) -> (user, password) mappings. A realm of None is considered a catch-all realm, which is searched if no other realm fits.

class urllib2.AbstractBasicAuthHandler([password_mgr])

This is a mixin class that helps with HTTP authentication, both to the remote host and to a proxy. password_mgr, if given, should be something that is compatible with HTTPPasswordMgr; refer to section HTTPPasswordMgr Objects for information on the interface that must be supported.

class urllib2.HTTPBasicAuthHandler([password_mgr])

Handle authentication with the remote host. password_mgr, if given, should be something that is compatible with HTTPPasswordMgr; refer to section HTTPPasswordMgr Objects for information on the interface that must be supported.

class urllib2.ProxyBasicAuthHandler([password_mgr])

Handle authentication with the proxy. password_mgr, if given, should be something that is compatible with HTTPPasswordMgr; refer to section HTTPPasswordMgr Objects for information on the interface that must be supported.

class urllib2.AbstractDigestAuthHandler([password_mgr])

This is a mixin class that helps with HTTP authentication, both to the remote host and to a proxy. password_mgr, if given, should be something that is compatible with HTTPPasswordMgr; refer to section HTTPPasswordMgr Objects for information on the interface that must be supported.

class urllib2.HTTPDigestAuthHandler([password_mgr])

Handle authentication with the remote host. password_mgr, if given, should be something that is compatible with HTTPPasswordMgr; refer to section HTTPPasswordMgr Objects for information on the interface that must be supported.

class urllib2.ProxyDigestAuthHandler([password_mgr])

Handle authentication with the proxy. password_mgr, if given, should be something that is compatible with HTTPPasswordMgr; refer to section HTTPPasswordMgr Objects for information on the interface that must be supported.

class urllib2.HTTPHandler

A class to handle opening of HTTP URLs.

class urllib2.HTTPSHandler([debuglevel[, context]])

A class to handle opening of HTTPS URLs. context has the same meaning as for httplib.HTTPSConnection.

Changed in version 2.7.9: context added.

class urllib2.FileHandler

Open local files.

class urllib2.FTPHandler

Open FTP URLs.

class urllib2.CacheFTPHandler

Open FTP URLs, keeping a cache of open FTP connections to minimize delays.

class urllib2.UnknownHandler

A catch-all class to handle unknown URLs.

class urllib2.HTTPErrorProcessor

Process HTTP error responses.

20.6.1. Request Objects

The following methods describe all of Request’s public interface, and so all must be overridden in subclasses.

Request.add_data(data)

Set the Request data to data. This is ignored by all handlers except HTTP handlers — and there it should be a byte string, and will change the request to be POST rather than GET.

Request.get_method()

Return a string indicating the HTTP request method. This is only meaningful for HTTP requests, and currently always returns 'GET' or 'POST'.

Request.has_data()

Return whether the instance has a non-None data.

Request.get_data()

Return the instance’s data.

Request.add_header(key, val)

Add another header to the request. Headers are currently ignored by all handlers except HTTP handlers, where they are added to the list of headers sent to the server. Note that there cannot be more than one header with the same name, and later calls will overwrite previous calls in case the key collides. Currently, this is no loss of HTTP functionality, since all headers which have meaning when used more than once have a (header-specific) way of gaining the same functionality using only one header.

Request.add_unredirected_header(key, header)

Add a header that will not be added to a redirected request.

New in version 2.4.

Request.has_header(header)

Return whether the instance has the named header (checks both regular and unredirected).

New in version 2.4.

Request.get_full_url()

Return the URL given in the constructor.

Request.get_type()

Return the type of the URL — also known as the scheme.

Request.get_host()

Return the host to which a connection will be made.

Request.get_selector()

Return the selector — the part of the URL that is sent to the server.

Request.get_header(header_name, default=None)

Return the value of the given header. If the header is not present, return the default value.

Request.header_items()

Return a list of tuples (header_name, header_value) of the Request headers.

Request.set_proxy(host, type)

Prepare the request by connecting to a proxy server. The host and type will replace those of the instance, and the instance’s selector will be the original URL given in the constructor.

Request.get_origin_req_host()

Return the request-host of the origin transaction, as defined by RFC 2965. See the documentation for the Request constructor.

Request.is_unverifiable()

Return whether the request is unverifiable, as defined by RFC 2965. See the documentation for the Request constructor.

20.6.2. OpenerDirector Objects

OpenerDirector instances have the following methods:

OpenerDirector.add_handler(handler)

handler should be an instance of BaseHandler. The following methods are searched, and added to the possible chains (note that HTTP errors are a special case).

• protocol_open — signal that the handler knows how to open protocol URLs.
• http_error_type — signal that the handler knows how to handle HTTP errors with HTTP error code type.
• protocol_error — signal that the handler knows how to handle errors from (non-http) protocol.
• protocol_request — signal that the handler knows how to pre-process protocol requests.
• protocol_response — signal that the handler knows how to post-process protocol responses.

OpenerDirector.open(url[, data][, timeout])

Open the given url (which can be a request object or a string), optionally passing the given data. Arguments, return values and exceptions raised are the same as those of urlopen() (which simply calls the open() method on the currently installed global OpenerDirector). The optional timeout parameter specifies a timeout in seconds for blocking operations like the connection attempt (if not specified, the global default timeout setting will be used). The timeout feature actually works only for HTTP, HTTPS and FTP connections).

Changed in version 2.6: timeout was added.

OpenerDirector.error(proto[, arg[, ...]])

Handle an error of the given protocol. This will call the registered error handlers for the given protocol with the given arguments (which are protocol specific). The HTTP protocol is a special case which uses the HTTP response code to determine the specific error handler; refer to the http_error_*() methods of the handler classes.

Return values and exceptions raised are the same as those of urlopen().

OpenerDirector objects open URLs in three stages:

The order in which these methods are called within each stage is determined by sorting the handler instances.

1. Every handler with a method named like protocol_request has that method called to pre-process the request.

2. Handlers with a method named like protocol_open are called to handle the request. This stage ends when a handler either returns a non-None value (ie. a response), or raises an exception (usually URLError). Exceptions are allowed to propagate.

   In fact, the above algorithm is first tried for methods named default_open(). If all such methods return None, the algorithm is repeated for methods named like protocol_open. If all such methods return None, the algorithm is repeated for methods named unknown_open().

   Note that the implementation of these methods may involve calls of the parent OpenerDirector instance’s open() and error() methods.

3. Every handler with a method named like protocol_response has that method called to post-process the response.

20.6.3. BaseHandler Objects

BaseHandler objects provide a couple of methods that are directly useful, and others that are meant to be used by derived classes. These are intended for direct use:

BaseHandler.add_parent(director)

Add a director as parent.

BaseHandler.close()

Remove any parents.

The following attributes and methods should only be used by classes derived from BaseHandler.

Note

 

The convention has been adopted that subclasses defining protocol_request() or protocol_response() methods are named *Processor; all others are named *Handler.

BaseHandler.parent

A valid OpenerDirector, which can be used to open using a different protocol, or handle errors.

BaseHandler.default_open(req)

This method is not defined in BaseHandler, but subclasses should define it if they want to catch all URLs.

This method, if implemented, will be called by the parent OpenerDirector. It should return a file-like object as described in the return value of the open() of OpenerDirector, or None. It should raise URLError, unless a truly exceptional thing happens (for example, MemoryError should not be mapped to URLError).

This method will be called before any protocol-specific open method.

BaseHandler.protocol_open(req)

(“protocol” is to be replaced by the protocol name.)

This method is not defined in BaseHandler, but subclasses should define it if they want to handle URLs with the given protocol.

This method, if defined, will be called by the parent OpenerDirector. Return values should be the same as for default_open().

BaseHandler.unknown_open(req)

This method is not defined in BaseHandler, but subclasses should define it if they want to catch all URLs with no specific registered handler to open it.

This method, if implemented, will be called by the parent OpenerDirector. Return values should be the same as for default_open().

BaseHandler.http_error_default(req, fp, code, msg, hdrs)

This method is not defined in BaseHandler, but subclasses should override it if they intend to provide a catch-all for otherwise unhandled HTTP errors. It will be called automatically by the OpenerDirector getting the error, and should not normally be called in other circumstances.

req will be a Request object, fp will be a file-like object with the HTTP error body, code will be the three-digit code of the error, msg will be the user-visible explanation of the code and hdrs will be a mapping object with the headers of the error.

Return values and exceptions raised should be the same as those of urlopen().

BaseHandler.http_error_nnn(req, fp, code, msg, hdrs)

nnn should be a three-digit HTTP error code. This method is also not defined in BaseHandler, but will be called, if it exists, on an instance of a subclass, when an HTTP error with code nnn occurs.

Subclasses should override this method to handle specific HTTP errors.

Arguments, return values and exceptions raised should be the same as for http_error_default().

BaseHandler.protocol_request(req)

(“protocol” is to be replaced by the protocol name.)

This method is not defined in BaseHandler, but subclasses should define it if they want to pre-process requests of the given protocol.

This method, if defined, will be called by the parent OpenerDirector. req will be a Request object. The return value should be aRequest object.

BaseHandler.protocol_response(req, response)

(“protocol” is to be replaced by the protocol name.)

This method is not defined in BaseHandler, but subclasses should define it if they want to post-process responses of the given protocol.

This method, if defined, will be called by the parent OpenerDirector. req will be a Request object. response will be an object implementing the same interface as the return value of urlopen(). The return value should implement the same interface as the return value of urlopen().

20.6.4. HTTPRedirectHandler Objects

Note

 

Some HTTP redirections require action from this module’s client code. If this is the case, HTTPError is raised. See RFC 2616 for details of the precise meanings of the various redirection codes.

HTTPRedirectHandler.redirect_request(req, fp, code, msg, hdrs, newurl)

Return a Request or None in response to a redirect. This is called by the default implementations of the http_error_30*() methods when a redirection is received from the server. If a redirection should take place, return a new Request to allow http_error_30*() to perform the redirect to newurl. Otherwise, raise HTTPError if no other handler should try to handle this URL, or return None if you can’t but another handler might.

Note

 

The default implementation of this method does not strictly follow RFC 2616, which says that 301 and 302 responses toPOST requests must not be automatically redirected without confirmation by the user. In reality, browsers do allow automatic redirection of these responses, changing the POST to a GET, and the default implementation reproduces this behavior.

HTTPRedirectHandler.http_error_301(req, fp, code, msg, hdrs)

Redirect to the Location: or URI: URL. This method is called by the parent OpenerDirector when getting an HTTP ‘moved permanently’ response.

HTTPRedirectHandler.http_error_302(req, fp, code, msg, hdrs)

The same as http_error_301(), but called for the ‘found’ response.

HTTPRedirectHandler.http_error_303(req, fp, code, msg, hdrs)

The same as http_error_301(), but called for the ‘see other’ response.

HTTPRedirectHandler.http_error_307(req, fp, code, msg, hdrs)

The same as http_error_301(), but called for the ‘temporary redirect’ response.

20.6.5. HTTPCookieProcessor Objects

New in version 2.4.

HTTPCookieProcessor instances have one attribute:

HTTPCookieProcessor.cookiejar

The cookielib.CookieJar in which cookies are stored.

20.6.6. ProxyHandler Objects

ProxyHandler.protocol_open(request)

(“protocol” is to be replaced by the protocol name.)

The ProxyHandler will have a method protocol_open for every protocol which has a proxy in the proxies dictionary given in the constructor. The method will modify requests to go through the proxy, by calling request.set_proxy(), and call the next handler in the chain to actually execute the protocol.

20.6.7. HTTPPasswordMgr Objects

These methods are available on HTTPPasswordMgr and HTTPPasswordMgrWithDefaultRealm objects.

HTTPPasswordMgr.add_password(realm, uri, user, passwd)

uri can be either a single URI, or a sequence of URIs. realm, user and passwd must be strings. This causes (user, passwd) to be used as authentication tokens when authentication for realm and a super-URI of any of the given URIs is given.

HTTPPasswordMgr.find_user_password(realm, authuri)

Get user/password for given realm and URI, if any. This method will return (None, None) if there is no matching user/password.

For HTTPPasswordMgrWithDefaultRealm objects, the realm None will be searched if the given realm has no matching user/password.

20.6.8. AbstractBasicAuthHandler Objects

AbstractBasicAuthHandler.http_error_auth_reqed(authreq, host, req, headers)

Handle an authentication request by getting a user/password pair, and re-trying the request. authreq should be the name of the header where the information about the realm is included in the request, host specifies the URL and path to authenticate for, req should be the (failed) Request object, and headers should be the error headers.

host is either an authority (e.g. "python.org") or a URL containing an authority component (e.g. "http://python.org/"). In either case, the authority must not contain a userinfo component (so, "python.org" and "python.org:80" are fine, "joe:password@python.org" is not).

20.6.9. HTTPBasicAuthHandler Objects

HTTPBasicAuthHandler.http_error_401(req, fp, code, msg, hdrs)

Retry the request with authentication information, if available.

20.6.10. ProxyBasicAuthHandler Objects

ProxyBasicAuthHandler.http_error_407(req, fp, code, msg, hdrs)

Retry the request with authentication information, if available.

20.6.11. AbstractDigestAuthHandler Objects

AbstractDigestAuthHandler.http_error_auth_reqed(authreq, host, req, headers)

authreq should be the name of the header where the information about the realm is included in the request, host should be the host to authenticate to, req should be the (failed) Request object, and headers should be the error headers.

20.6.12. HTTPDigestAuthHandler Objects

HTTPDigestAuthHandler.http_error_401(req, fp, code, msg, hdrs)

Retry the request with authentication information, if available.

20.6.13. ProxyDigestAuthHandler Objects

ProxyDigestAuthHandler.http_error_407(req, fp, code, msg, hdrs)

Retry the request with authentication information, if available.

20.6.14. HTTPHandler Objects

HTTPHandler.http_open(req)

Send an HTTP request, which can be either GET or POST, depending on req.has_data().

20.6.15. HTTPSHandler Objects

HTTPSHandler.https_open(req)

Send an HTTPS request, which can be either GET or POST, depending on req.has_data().

20.6.16. FileHandler Objects

FileHandler.file_open(req)

Open the file locally, if there is no host name, or the host name is 'localhost'. Change the protocol to ftp otherwise, and retry opening it using parent.

20.6.17. FTPHandler Objects

FTPHandler.ftp_open(req)

Open the FTP file indicated by req. The login is always done with empty username and password.

20.6.18. CacheFTPHandler Objects

CacheFTPHandler objects are FTPHandler objects with the following additional methods:

CacheFTPHandler.setTimeout(t)

Set timeout of connections to t seconds.

CacheFTPHandler.setMaxConns(m)

Set maximum number of cached connections to m.

20.6.19. UnknownHandler Objects

UnknownHandler.unknown_open()

Raise a URLError exception.

20.6.20. HTTPErrorProcessor Objects

New in version 2.4.

HTTPErrorProcessor.http_response()

Process HTTP error responses.

For 200 error codes, the response object is returned immediately.

For non-200 error codes, this simply passes the job on to the protocol_error_code handler methods, via OpenerDirector.error(). Eventually,urllib2.HTTPDefaultErrorHandler will raise an HTTPError if no other handler handles the error.

HTTPErrorProcessor.https_response()

Process HTTPS error responses.

The behavior is same as http_response().

20.6.21. Examples

In addition to the examples below, more examples are given in HOWTO Fetch Internet Resources Using urllib2.

This example gets the python.org main page and displays the first 100 bytes of it:

>>> import urllib2
>>> f = urllib2.urlopen('http://www.python.org/')
>>> print f.read(100)
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<?xml-stylesheet href="./css/ht2html

Here we are sending a data-stream to the stdin of a CGI and reading the data it returns to us. Note that this example will only work when the Python installation supports SSL.

>>> import urllib2
>>> req = urllib2.Request(url='https://localhost/cgi-bin/test.cgi',
...                       data='This data is passed to stdin of the CGI')
>>> f = urllib2.urlopen(req)
>>> print f.read()
Got Data: "This data is passed to stdin of the CGI"

The code for the sample CGI used in the above example is:

#!/usr/bin/env python
import sys
data = sys.stdin.read()
print 'Content-type: text-plain\n\nGot Data: "%s"' % data

Use of Basic HTTP Authentication:

import urllib2
# Create an OpenerDirector with support for Basic HTTP Authentication...
auth_handler = urllib2.HTTPBasicAuthHandler()
auth_handler.add_password(realm='PDQ Application',
                          uri='https://mahler:8092/site-updates.py',
                          user='klem',
                          passwd='kadidd!ehopper')
opener = urllib2.build_opener(auth_handler)
# ...and install it globally so it can be used with urlopen.
urllib2.install_opener(opener)
urllib2.urlopen('http://www.example.com/login.html')

build_opener() provides many handlers by default, including a ProxyHandler. By default, ProxyHandler uses the environment variables named <scheme>_proxy, where <scheme> is the URL scheme involved. For example, the http_proxy environment variable is read to obtain the HTTP proxy’s URL.

This example replaces the default ProxyHandler with one that uses programmatically-supplied proxy URLs, and adds proxy authorization support with ProxyBasicAuthHandler.

proxy_handler = urllib2.ProxyHandler({'http': 'http://www.example.com:3128/'})
proxy_auth_handler = urllib2.ProxyBasicAuthHandler()
proxy_auth_handler.add_password('realm', 'host', 'username', 'password')

opener = urllib2.build_opener(proxy_handler, proxy_auth_handler)
# This time, rather than install the OpenerDirector, we use it directly:
opener.open('http://www.example.com/login.html')

Adding HTTP headers:

Use the headers argument to the Request constructor, or:

import urllib2
req = urllib2.Request('http://www.example.com/')
req.add_header('Referer', 'http://www.python.org/')
# Customize the default User-Agent header value:
req.add_header('User-Agent', 'urllib-example/0.1 (Contact: . . .)')
r = urllib2.urlopen(req)

OpenerDirector automatically adds a User-Agent header to every Request. To change this:

import urllib2
opener = urllib2.build_opener()
opener.addheaders = [('User-agent', 'Mozilla/5.0')]
opener.open('http://www.example.com/')

Also, remember that a few standard headers (Content-Length, Content-Type and Host) are added when the Request is passed to urlopen() (or OpenerDirector.open()).