HOWTO 使用 urllib 包获取网络资源
********************************

作者:
   Michael Foord

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
====


Related Articles
^^^^^^^^^^^^^^^^

关于使用 Python 获取网页资源，你或许还可以找到下列有用的文章：

* 基本的验证

     关于 *基本的验证* 的入门指南，带有一些 Python 的示例。

**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
----

有时候你想要给一个 URL 发送数据（通常这个URL指向一个CGI（通用网关接口
）脚本或者其他 web 应用）。 对于 HTTP，这通常使用一个 **POST** 请求来
完成。 比如在浏览器上提交一个 HTML 表单。 但并不是所有的 POST 都来自表
单：你能使用一个 POST 来传输任何数据到你自己的应用上。 在使用常见的
HTML 表单的情况下，数据需要以标准的方式编码，然后再作为 "data" 参数传
给 Request 对象。 编码需要使用一个来自 "urllib.parse" 库的函数。

   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.
