Como buscar recursos da Internet usando o pacote urllib¶
- Autor:
Introdução¶
urllib.request é um modulo Python para buscar URLs (Uniform Resource Locators). Ele oferece uma interface muito simples, na forma da função urlopen. Este é capaz de buscar URLs usando uma variedade de diferentes protocolos. Ele também oferece uma interface um pouco mais complexa para lidar com situações comuns - como autenticação básica, cookies, proxies e assim por diante. Estes são fornecidos por objetos chamados handlers (manipuladores) e openers (abridores).
urllib.request suporta o acesso a URLs por meio de vários “esquemas de URL” (identificados pela string antes de ":" na URL - por exemplo "ftp" é o esquema de URL em "ftp://python.org/") usando o protocolo de rede associado a ele (como FTP e HTTP). Este tutorial foca no caso mais comum, HTTP.
Para situações simples “urlopen” é muito fácil de usar. Mas assim que você se depara com erros ou casos não triviais ao abrir URLs HTTP, você vai precisar entender um pouco mais do HyperText Transfer Protocol. A literatura de referência mais reconhecida e compreensível para o HTTP é RFC 2616. Ela é um documento técnico e não foi feita para ser fácil de ler. Este documento busca ilustrar o uso de urllib com detalhes suficientes sobre HTTP para te permitir seguir adiante. Ele não tem a intenção de substituir a documentação do urllib.request, mas é suplementar a ela.
Acessando URLs¶
O modo mais simples de usar urllib.request é o seguinte:
import urllib.request
with urllib.request.urlopen('http://python.org/') as response:
html = response.read()
Se você deseja obter um recurso via URL e guardá-lo em uma localização temporária, você pode fazê-lo com as funções shutil.copyfileobj() e tempfile.NamedTemporaryFile():
import shutil
import tempfile
import urllib.request
with urllib.request.urlopen('http://python.org/') as response:
with tempfile.NamedTemporaryFile(delete=False) as tmp_file:
shutil.copyfileobj(response, tmp_file)
with open(tmp_file.name) as html:
pass
Muitos usos de urllib são simples assim (repare que ao invés de uma URL ‘http:’ nós poderíamos ter usado uma string URL começando com ‘ftp:’, ‘file:’, etc.).. No entanto, o propósito deste tutorial é explicar casos mais complicados, concentrando em HTTP.
HTTP é baseado em solicitações (requests) e respostas (responses) - o cliente faz solicitações e os servidores mandam respostas. urllib.request espelha isto com um objeto Request que representa a solicitação HTTP que você está fazendo. Na sua forma mais simples, você cria um objeto Request que especifica a URL que você quer acessar. Chamar urlopen com este objeto Request retorna um objeto de resposta para a URL solicitada. Essa resposta é um objeto arquivo ou similar, o que significa que você pode, por exemplo, chamar .read() na resposta:
import urllib.request
req = urllib.request.Request('http://python.org/')
with urllib.request.urlopen(req) as response:
the_page = response.read()
Note que urllib.request usa a mesma interface Request para tratar todos os esquemas URL. Por exemplo, você pode fazer uma solicitação FTP da seguinte forma:
req = urllib.request.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 about the 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 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.parse
library.
import urllib.parse
import urllib.request
url = 'http://www.someserver.com/cgi-bin/register.cgi'
values = {'name' : 'Michael Foord',
'location' : 'Northampton',
'language' : 'Python' }
data = urllib.parse.urlencode(values)
data = data.encode('ascii') # data should be bytes
req = urllib.request.Request(url, data)
with urllib.request.urlopen(req) as response:
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, urllib 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.
Isso é feito como abaixo:
>>> import urllib.request
>>> import urllib.parse
>>> data = {}
>>> data['name'] = 'Somebody Here'
>>> data['location'] = 'Northampton'
>>> data['language'] = 'Python'
>>> url_values = urllib.parse.urlencode(data)
>>> print(url_values) # The order may differ from below.
name=Somebody+Here&language=Python&location=Northampton
>>> url = 'http://www.example.com/example.cgi'
>>> full_url = url + '?' + url_values
>>> data = urllib.request.urlopen(full_url)
Observe que o URL completo é criado adicionando um ? ao URL, seguido pelos valores codificados.
Headers¶
We’ll discuss here one particular HTTP header, to illustrate how to add headers to your HTTP request.
Some websites [1] dislike being browsed by programs, or send different versions
to different browsers [2]. By default urllib 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 [3]. 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 [4].
import urllib.parse
import urllib.request
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.parse.urlencode(values)
data = data.encode('ascii')
req = urllib.request.Request(url, data, headers)
with urllib.request.urlopen(req) as response:
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.
Tratamento de exceções¶
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.
The exception classes are exported from the urllib.error module.
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 = urllib.request.Request('http://www.pretend_server.org')
>>> try: urllib.request.urlopen(req)
... except urllib.error.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, urllib 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.
http.server.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 as returned by the urllib.response module:
>>> req = urllib.request.Request('http://www.python.org/fish.html')
>>> try:
... urllib.request.urlopen(req)
... except urllib.error.HTTPError as e:
... print(e.code)
... print(e.read())
...
404
b'<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">\n\n\n<html
...
<title>Page Not Found</title>\n
...
Wrapping it Up¶
So if you want to be prepared for HTTPError or URLError there are two
basic approaches. I prefer the second approach.
Number 1¶
from urllib.request import Request, urlopen
from urllib.error import URLError, HTTPError
req = Request(someurl)
try:
response = urlopen(req)
except HTTPError as e:
print('The server couldn\'t fulfill the request.')
print('Error code: ', e.code)
except URLError as e:
print('We failed to reach a server.')
print('Reason: ', e.reason)
else:
# everything is fine
Nota
The except HTTPError must come first, otherwise except URLError
will also catch an HTTPError.
Number 2¶
from urllib.request import Request, urlopen
from urllib.error import URLError
req = Request(someurl)
try:
response = urlopen(req)
except URLError as e:
if hasattr(e, 'reason'):
print('We failed to reach a server.')
print('Reason: ', e.reason)
elif hasattr(e, 'code'):
print('The server couldn\'t fulfill the request.')
print('Error code: ', e.code)
else:
# everything is fine
info and geturl¶
The response returned by urlopen (or the HTTPError instance) has two
useful methods info() and geturl() and is defined in the module
urllib.response.
geturl - this returns the real URL of the page fetched. This is useful because
urlopen(or the opener 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 currently an
http.client.HTTPMessageinstance.
Typical headers include ‘Content-length’, ‘Content-type’, and so on. See the Quick Reference to HTTP Headers for a useful listing of HTTP headers with brief explanations of their meaning and use.
Openers and Handlers¶
When you fetch a URL you use an opener (an instance of the perhaps
confusingly named urllib.request.OpenerDirector). Normally we have been using
the default opener - via urlopen - but you can create custom
openers. Openers use handlers. All the “heavy lifting” is done by the
handlers. Each handler knows how to open URLs for a particular URL scheme (http,
ftp, etc.), or how to handle an aspect of URL opening, for example HTTP
redirections or HTTP cookies.
You will want to create openers if you want to fetch URLs with specific handlers installed, for example to get an opener that handles cookies, or to get an opener that does not handle redirections.
To create an opener, instantiate an OpenerDirector, and then call
.add_handler(some_handler_instance) repeatedly.
Alternatively, you can use build_opener, which is a convenience function for
creating opener objects with a single function call. build_opener adds
several handlers by default, but provides a quick way to add more and/or
override the default handlers.
Other sorts of handlers you might want to can handle proxies, authentication, and other common but slightly specialised situations.
install_opener can be used to make an opener object the (global) default
opener. This means that calls to urlopen will use the opener you have
installed.
Opener objects have an open method, which can be called directly to fetch
urls in the same way as the urlopen function: there’s no need to call
install_opener, except as a convenience.
Basic Authentication¶
To illustrate creating and installing a handler we will use the
HTTPBasicAuthHandler. For a more detailed discussion of this subject –
including an explanation of how Basic Authentication works - see the Basic
Authentication Tutorial.
When authentication is required, the server sends a header (as well as the 401
error code) requesting authentication. This specifies the authentication scheme
and a ‘realm’. The header looks like: WWW-Authenticate: SCHEME
realm="REALM".
e.g.
WWW-Authenticate: Basic realm="cPanel Users"
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.
The HTTPBasicAuthHandler uses an object called a password manager to handle
the mapping of URLs 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. Frequently one doesn’t care what the realm is. In that
case, it is convenient to use HTTPPasswordMgrWithDefaultRealm. This allows
you to specify a default username and password for a URL. This will be supplied
in the absence of you providing an alternative combination for a specific
realm. We indicate this by providing None as the realm argument to the
add_password method.
The top-level URL is the first URL that requires authentication. URLs “deeper” than the URL you pass to .add_password() will also match.
# create a password manager
password_mgr = urllib.request.HTTPPasswordMgrWithDefaultRealm()
# Add the username and password.
# If we knew the realm, we could use it instead of None.
top_level_url = "http://example.com/foo/"
password_mgr.add_password(None, top_level_url, username, password)
handler = urllib.request.HTTPBasicAuthHandler(password_mgr)
# create "opener" (OpenerDirector instance)
opener = urllib.request.build_opener(handler)
# use the opener to fetch a URL
opener.open(a_url)
# Install the opener.
# Now all calls to urllib.request.urlopen use our opener.
urllib.request.install_opener(opener)
Nota
In the above example we only supplied our HTTPBasicAuthHandler to
build_opener. By default openers have the handlers for normal situations
– ProxyHandler (if a proxy setting such as an http_proxy
environment variable is set), UnknownHandler, HTTPHandler,
HTTPDefaultErrorHandler, HTTPRedirectHandler, FTPHandler,
FileHandler, DataHandler, HTTPErrorProcessor.
top_level_url is in fact either a full URL (including the ‘http:’ scheme
component and the hostname and optionally the port number)
e.g. "http://example.com/" or an “authority” (i.e. the hostname,
optionally including the port number) e.g. "example.com" or "example.com:8080"
(the latter example includes a port number). The authority, if present, must
NOT contain the “userinfo” component - for example "joe:password@example.com" is
not correct.
Proxies¶
urllib will auto-detect your proxy settings and use those. This is through
the ProxyHandler, which is part of the normal handler chain when a proxy
setting is detected. Normally that’s a good thing, but there are occasions
when it may not be helpful [5]. One way to do this is to setup our own
ProxyHandler, with no proxies defined. This is done using similar steps to
setting up a Basic Authentication handler:
>>> proxy_support = urllib.request.ProxyHandler({})
>>> opener = urllib.request.build_opener(proxy_support)
>>> urllib.request.install_opener(opener)
Nota
Currently urllib.request does not support fetching of https locations
through a proxy. However, this can be enabled by extending urllib.request as
shown in the recipe [6].
Nota
HTTP_PROXY will be ignored if a variable REQUEST_METHOD is set; see
the documentation on getproxies().
Sockets and Layers¶
The Python support for fetching resources from the web is layered. urllib uses
the http.client library, which in turn uses the socket library.
As of Python 2.3 you can specify how long a socket should wait for a response before timing out. This can be useful in applications which have to fetch web pages. By default the socket module has no timeout and can hang. Currently, the socket timeout is not exposed at the http.client or urllib.request levels. However, you can set the default timeout globally for all sockets using
import socket
import urllib.request
# timeout in seconds
timeout = 10
socket.setdefaulttimeout(timeout)
# this call to urllib.request.urlopen now uses the default timeout
# we have set in the socket module
req = urllib.request.Request('http://www.voidspace.org.uk')
response = urllib.request.urlopen(req)
Notas de rodapé¶
This document was reviewed and revised by John Lee.