HOWTO 使用 urllib 包获取网络资源¶
注解
这份 HOWTO 文档的早期版本有一份法语的译文,可在 urllib2 - Le Manuel manquant 处查阅。
概述¶
urllib.request 是一个用于获取 URL (统一资源定位地址)的 Python 模块。它以 urlopen 函数的形式提供了一个非常简单的接口。该接口能够使用不同的协议获取 URL。同时它也提供了一个略微复杂的接口来处理常见情形——如:基本验证、cookies、代理等等。这些功能是通过叫做 handlers 和 opener 的对象来提供的。
urllib.request 支持多种 "URL 网址方案" (通过 URL中 ":"
之前的字符串加以区分——如 URL 地址 "ftp://python.org/"` 中的 ``"ftp"`
) ,使用与之相关的网络协议(如:FTP、 HTTP)来获取 URL 资源。本指南重点关注最常用的情形—— 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 urllib,
with enough detail about HTTP to help you through. It is not intended to replace
the urllib.request
docs, but is supplementary to them.
提取URL¶
下面是使用 urllib.request 最简单的方式:
import urllib.request
with urllib.request.urlopen('http://python.org/') as response:
html = response.read()
如果你想通过 URL 获取资源并保存某个临时的地方,你可以通过 shutil.copyfileobj()
和 tempfile.NamedTemporaryFile()
函数:
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
urllib很易于使用(注意URL不仅仅可以以'http:'开头,也可以是'ftp:','file:'等)。但是,这篇教程的目的是介绍更加复杂的用法,大多数是以HTTP举例。
HTTP基于请求和回应——客户端像服务器请求,服务器回应。urllib.request将你的HTTP请求保存为一个``Request``对象。在最简单的情况下,一个Request对象里包含你所请求的特定URL。以当前的Request对象作为参数调用``urlopen``返回服务器对你正在请求的URL的回应。回应是个文件类对象,所以你可以调用如``.read()``等命令。
import urllib.request
req = urllib.request.Request('http://www.voidspace.org.uk')
with urllib.request.urlopen(req) as response:
the_page = response.read()
注意urllib.request中的Request接口也支持处理所有的协议。比如,你可以像这样做一个 FTP 请求:
req = urllib.request.Request('ftp://example.com/')
在 HTTP 的情况下,Request 对象允许你做两件额外的事:一,你可以向服务器发送数据。二,你可以向服务器发送额外的信息(“元数据”): 关于 数据或请求本身的。信息将以“HTTP头”的方式发过去。让我们一个个看过去。
数据¶
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.
具体操作如下:
>>> 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)
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 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.
处理异常¶
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.
异常类从 urllib.error
模块中导出。
URLError¶
通常,引发 URLError 的原因是没有网络连接(或者没有到指定服务器的路由),或者指定的服务器不存在。该情况下,将会引发该异常,并带有一个 'reason' 属性,该属性是一个包含错误代码和文本错误信息的元组。
例如
>>> 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¶
从服务器返回的每个 HTTP 响应都包含一个数字的 “状态码”。有时该状态码表明服务器无法完成该请求。默认的处理器(函数?)将会为你处理这其中的一些响应。(例如,如果响应包含了 "redirection",将会要求客户端去向另外的 URL 获取文档,urllib 将会为你处理该情形)。对于那些它无法处理的(状态代码),urlopen 将会引发一个 HTTPError
。典型的错误包括:‘404’(页面无法找到)、‘403’(请求遭拒绝)和 ’401‘ (需要身份验证)。
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.
错误代码¶
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
...
包装起来¶
So if you want to be prepared for HTTPError
or URLError
there are two
basic approaches. I prefer the second approach.
数字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
注解
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¶
由 urlopen (或者 HTTPError
实例)所返回的响应包含两个有用的方法: info()
和 geturl()
,该响应由模块 urllib.response
定义。
geturl - 返回所获取页面的真实 URL。该方法很有用,因为 urlopen
(或者所使用的 opener 对象)可能回包括一次重定向。所获取页面的 URL 未必就是所请求的 URL 。
info - 该方法返回一个类似字典的对象,描述了所获取的页面,特别是由服务器送出的头部信息(headers) 。目前它是一个 http.client.HTTPMessage
实例。
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.
基本认证¶
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"
.
例如
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)
注解
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.
代理¶
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)
注解
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.
注解
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)
备注¶
这篇文档由 John Lee 审订。
- 1
例如 Google。
- 2
Browser sniffing is a very bad practice for website design - building sites using web standards is much more sensible. Unfortunately a lot of sites still send different versions to different browsers.
- 3
The user agent for MSIE 6 is 'Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; .NET CLR 1.1.4322)'
- 4
For details of more HTTP request headers, see Quick Reference to HTTP Headers.
- 5
In my case I have to use a proxy to access the internet at work. If you attempt to fetch localhost URLs through this proxy it blocks them. IE is set to use the proxy, which urllib picks up on. In order to test scripts with a localhost server, I have to prevent urllib from using the proxy.
- 6
urllib opener for SSL proxy (CONNECT method): ASPN Cookbook Recipe.