HOWTO Fetch Internet Resources Using urllib2 ******************************************** Author: Michael Foord Note: There is a French translation of an earlier revision of this HOWTO, available at urllib2 - Le Manuel manquant. Introduction ============ Related Articles ^^^^^^^^^^^^^^^^ You may also find useful the following article on fetching web resources with Python: * Basic Authentication A tutorial on *Basic Authentication*, with examples in Python. **urllib2** is a Python module for fetching URLs (Uniform Resource Locators). It offers a very simple interface, in the form of the *urlopen* function. This is capable of fetching URLs using a variety of different protocols. It also offers a slightly more complex interface for handling common situations - like basic authentication, cookies, proxies and so on. These are provided by objects called handlers and openers. urllib2 supports fetching URLs for many “URL schemes” (identified by the string before the "":"" in URL - for example ""ftp"" is the URL scheme of ""ftp://python.org/"") using their associated network protocols (e.g. FTP, HTTP). This tutorial focuses on the most common case, HTTP. For straightforward situations *urlopen* is very easy to use. But as soon as you encounter errors or non-trivial cases when opening HTTP URLs, you will need some understanding of the HyperText Transfer Protocol. The most comprehensive and authoritative reference to HTTP is **RFC 2616**. This is a technical document and not intended to be easy to read. This HOWTO aims to illustrate using *urllib2*, with enough detail about HTTP to help you through. It is not intended to replace the "urllib2" docs, but is supplementary to them. Fetching URLs ============= The simplest way to use urllib2 is as follows: import urllib2 response = urllib2.urlopen('http://python.org/') html = response.read() Many uses of urllib2 will be that simple (note that instead of an ‘http:’ URL we could have used a URL starting with ‘ftp:’, ‘file:’, etc.). However, it’s the purpose of this tutorial to explain the more complicated cases, concentrating on HTTP. HTTP is based on requests and responses - the client makes requests and servers send responses. urllib2 mirrors this with a "Request" object which represents the HTTP request you are making. In its simplest form you create a Request object that specifies the URL you want to fetch. Calling "urlopen" with this Request object returns a response object for the URL requested. This response is a file-like object, which means you can for example call ".read()" on the response: import urllib2 req = urllib2.Request('http://www.voidspace.org.uk') response = urllib2.urlopen(req) the_page = response.read() Note that urllib2 makes use of the same Request interface to handle all URL schemes. For example, you can make an FTP request like so: req = urllib2.Request('ftp://example.com/') In the case of HTTP, there are two extra things that Request objects allow you to do: First, you can pass data to be sent to the server. Second, you can pass extra information (“metadata”) *about* the data or the about request itself, to the server - this information is sent as HTTP “headers”. Let’s look at each of these in turn. Data ---- Sometimes you want to send data to a URL (often the URL will refer to a CGI (Common Gateway Interface) script [1] or other web application). With HTTP, this is often done using what’s known as a **POST** request. This is often what your browser does when you submit a HTML form that you filled in on the web. Not all POSTs have to come from forms: you can use a POST to transmit arbitrary data to your own application. In the common case of HTML forms, the data needs to be encoded in a standard way, and then passed to the Request object as the "data" argument. The encoding is done using a function from the "urllib" library *not* from "urllib2". import urllib import urllib2 url = 'http://www.someserver.com/cgi-bin/register.cgi' values = {'name' : 'Michael Foord', 'location' : 'Northampton', 'language' : 'Python' } data = urllib.urlencode(values) req = urllib2.Request(url, data) response = urllib2.urlopen(req) the_page = response.read() Note that other encodings are sometimes required (e.g. for file upload from HTML forms - see HTML Specification, Form Submission for more details). If you do not pass the "data" argument, urllib2 uses a **GET** request. One way in which GET and POST requests differ is that POST requests often have “side-effects”: they change the state of the system in some way (for example by placing an order with the website for a hundredweight of tinned spam to be delivered to your door). Though the HTTP standard makes it clear that POSTs are intended to *always* cause side-effects, and GET requests *never* to cause side- effects, nothing prevents a GET request from having side-effects, nor a POST requests from having no side-effects. Data can also be passed in an HTTP GET request by encoding it in the URL itself. This is done as follows: >>> import urllib2 >>> import urllib >>> data = {} >>> data['name'] = 'Somebody Here' >>> data['location'] = 'Northampton' >>> data['language'] = 'Python' >>> url_values = urllib.urlencode(data) >>> print url_values # The order may differ. name=Somebody+Here&language=Python&location=Northampton >>> url = 'http://www.example.com/example.cgi' >>> full_url = url + '?' + url_values >>> data = urllib2.urlopen(full_url) Notice that the full URL is created by adding a "?" to the URL, followed by the encoded values. Headers ------- We’ll discuss here one particular HTTP header, to illustrate how to add headers to your HTTP request. Some websites [2] dislike being browsed by programs, or send different versions to different browsers [3]. By default urllib2 identifies itself as "Python-urllib/x.y" (where "x" and "y" are the major and minor version numbers of the Python release, e.g. "Python- urllib/2.5"), which may confuse the site, or just plain not work. The way a browser identifies itself is through the "User-Agent" header [4]. When you create a Request object you can pass a dictionary of headers in. The following example makes the same request as above, but identifies itself as a version of Internet Explorer [5]. import urllib import urllib2 url = 'http://www.someserver.com/cgi-bin/register.cgi' user_agent = 'Mozilla/5.0 (Windows NT 6.1; Win64; x64)' values = {'name': 'Michael Foord', 'location': 'Northampton', 'language': 'Python' } headers = {'User-Agent': user_agent} data = urllib.urlencode(values) req = urllib2.Request(url, data, headers) response = urllib2.urlopen(req) the_page = response.read() The response also has two useful methods. See the section on info and geturl which comes after we have a look at what happens when things go wrong. Handling Exceptions =================== *urlopen* raises "URLError" when it cannot handle a response (though as usual with Python APIs, built-in exceptions such as "ValueError", "TypeError" etc. may also be raised). "HTTPError" is the subclass of "URLError" raised in the specific case of HTTP URLs. URLError -------- Often, URLError is raised 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 urllib2.URLError as e: ... print e.reason ... (4, 'getaddrinfo failed') HTTPError --------- Every HTTP response from the server contains a numeric “status code”. Sometimes the status code indicates that the server is unable to fulfil the request. The default handlers will handle some of these responses for you (for example, if the response is a “redirection” that requests the client fetch the document from a different URL, urllib2 will handle that for you). For those it can’t handle, urlopen will raise an "HTTPError". Typical errors include ‘404’ (page not found), ‘403’ (request forbidden), and ‘401’ (authentication required). See section 10 of RFC 2616 for a reference on all the HTTP error codes. The "HTTPError" instance raised will have an integer ‘code’ attribute, which corresponds to the error sent by the server. Error Codes ~~~~~~~~~~~ Because the default handlers handle redirects (codes in the 300 range), and codes in the 100–299 range indicate success, you will usually only see error codes in the 400–599 range. "BaseHTTPServer.BaseHTTPRequestHandler.responses" is a useful dictionary of response codes in that shows all the response codes used by RFC 2616. The dictionary is reproduced here for convenience # Table mapping response codes to messages; entries have the # form {code: (shortmessage, longmessage)}. responses = { 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 Content', '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 Timeout', '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 Server 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 Unavailable', '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 response 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 urllib2.HTTPError as e: ... print e.code ... print e.read() ... 404 ...