howto-urllib2.pdf

HOWTOFetchInternetResources

Usingurllib2

Release2.7.6

GuidovanRossum

FredL.Drake,Jr.,editor

March 17, 2014

PythonSoftwareFoundation

Email:docs@python.org

Contents

1Introduction

2FetchingURLs

2.1

Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

iii

2.2

Headers

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

iii

3HandlingExceptions

3.1

URLError . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3.2

HTTPError

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Error Codes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3.3

Wrapping it Up

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Number 1

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Number 2

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

vii

4infoandgeturl

vii

5OpenersandHandlers

vii

6BasicAuthentication

viii

7Proxies

8SocketsandLayers

9Footnotes

Index xi

Author Michael Foord

Note: There is an French translation of an earlier revision of this HOWTO, available at urllib2 - Le Manuel

manquant .

1Introduction

You may also ﬁnd useful the following article on fetching web resources with Python:

• Basic Authentication

A tutorial onBasicAuthentication, with examples in Python.

urllib2is a Python module for fetching URLs (Uniform Resource Locators). It offers a very simple interface,

in the form of theurlopenfunction. 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 situationsurlopenis 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 RFC2616 . This is a technical document and not intended

to be easy to read. This HOWTO aims to illustrate usingurllib2, with enough detail about HTTP to help you

through. It is not intended to replace the urllib2 docs, but is supplementary to them.

2FetchingURLs

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 an URL start-

ing 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”)aboutthe 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.

2.1Data

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 aPOSTrequest. 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 librarynotfrom urllib2 .

import urllib

import urllib2

url = ’http://www.someserver.com/cgi-bin/register.cgi’

values = { ’name’ : ’MichaelFoord’ ,

’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 Speciﬁca-

tion, Form Submission for more details).

If you do not pass the data argument, urllib2 uses aGETrequest. 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 toalwayscause side-effects, and GET requests

neverto 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’ ] = ’SomebodyHere’

>>> data[ ’location’ ] = ’Northampton’

>>> data[ ’language’ ] = ’Python’

>>> url_values = urllib . urlencode(data)

>>> print url_values #Theordermaydiffer.

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.

2.2Headers

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

1 For an introduction to the CGI protocol see Writing Web Applications in Python .

2 Like Google for example. Theproperway to use google from a program is to use PyGoogle of course. See Voidspace Google for some

examples of using the Google API.

3 Browser snifﬁng is a very bad practise for website design - building sites using web standards is much more sensible. Unfortunately a lot

of sites still send different versions to different browsers.

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/4.0(compatible;MSIE5.5;WindowsNT)’

values = { ’name’ : ’MichaelFoord’ ,

’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.

3HandlingExceptions

urlopenraises URLError when it cannot handle a response (though as usual with Python APIs, built-in excep-

tions such as ValueError , TypeError etc. may also be raised).

HTTPError is the subclass of URLError raised in the speciﬁc case of HTTP URLs.

3.1URLError

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 URLError as e:

... print e . reason

...

(4,’getaddrinfofailed’)

3.2HTTPError

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.

The user agent for MSIE 6 is‘Mozilla/4.0(compatible;MSIE6.0;WindowsNT5.1;SV1;.NETCLR1.1.4322)’

For details of more HTTP request headers, see Quick Reference to HTTP Headers .

ErrorCodes

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

#Tablemappingresponsecodestomessages;entrieshavethe

#form{code:(shortmessage,longmessage)}.

responses = {

100 :( ’Continue’ , ’Requestreceived,pleasecontinue’ ),

101 :( ’SwitchingProtocols’ ,

’Switchingtonewprotocol;obeyUpgradeheader’ ),

200 :( ’OK’ , ’Requestfulfilled,documentfollows’ ),

201 :( ’Created’ , ’Documentcreated,URLfollows’ ),

202 :( ’Accepted’ ,

’Requestaccepted,processingcontinuesoff-line’ ),

203 :( ’Non-AuthoritativeInformation’ , ’Requestfulfilledfromcache’ ),

204 :( ’NoContent’ , ’Requestfulfilled,nothingfollows’ ),

205 :( ’ResetContent’ , ’Clearinputformforfurtherinput.’ ),

206 :( ’PartialContent’ , ’Partialcontentfollows.’ ),

300 :( ’MultipleChoices’ ,

’Objecthasseveralresources--seeURIlist’ ),

301 :( ’MovedPermanently’ , ’Objectmovedpermanently--seeURIlist’ ),

302 :( ’Found’ , ’Objectmovedtemporarily--seeURIlist’ ),

303 :( ’SeeOther’ , ’Objectmoved--seeMethodandURLlist’ ),

304 :( ’NotModified’ ,

’Documenthasnotchangedsincegiventime’ ),

305 :( ’UseProxy’ ,

’YoumustuseproxyspecifiedinLocationtoaccessthis’

’resource.’ ),

307 :( ’TemporaryRedirect’ ,

’Objectmovedtemporarily--seeURIlist’ ),

400 :( ’BadRequest’ ,

’Badrequestsyntaxorunsupportedmethod’ ),

401 :( ’Unauthorized’ ,

’Nopermission--seeauthorizationschemes’ ),

402 :( ’PaymentRequired’ ,

’Nopayment--seechargingschemes’ ),

403 :( ’Forbidden’ ,

’Requestforbidden--authorizationwillnothelp’ ),

404 :( ’NotFound’ , ’NothingmatchesthegivenURI’ ),

405 :( ’MethodNotAllowed’ ,

’Specifiedmethodisinvalidforthisserver.’ ),

406 :( ’NotAcceptable’ , ’URInotavailableinpreferredformat.’ ),

407 :( ’ProxyAuthenticationRequired’ , ’Youmustauthenticatewith’

’thisproxybeforeproceeding.’ ),

408 :( ’RequestTimeout’ , ’Requesttimedout;tryagainlater.’ ),

409 :( ’Conflict’ , ’Requestconflict.’ ),

410 :( ’Gone’ ,

’URInolongerexistsandhasbeenpermanentlyremoved.’ ),

411 :( ’LengthRequired’ , ’ClientmustspecifyContent-Length.’ ),

412 :( ’PreconditionFailed’ , ’Preconditioninheadersisfalse.’ ),

413 :( ’RequestEntityTooLarge’ , ’Entityistoolarge.’ ),

414 :( ’Request-URITooLong’ , ’URIistoolong.’ ),

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