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# HOWTO Fetch Internet Resources Using urllib2

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

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.

## 提取URL

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/')

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 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.

具体操作如下:

    >>> 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.

## 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.

### 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 = 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.

#### 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.

`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
    <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
    "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
    ...
    <title>Page Not Found</title>
    ...

### 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 urllib2 import Request, urlopen, 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 urllib2 import Request, urlopen, 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()`.

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 - this returns a dictionary-like object that describes the page fetched, particularly the headers sent by the server. It is currently an `httplib.HTTPMessage` instance.

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 `urllib2.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 = urllib2.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 = urllib2.HTTPBasicAuthHandler(password_mgr)

    # create "opener" (OpenerDirector instance)
    opener = urllib2.build_opener(handler)

    # use the opener to fetch a URL
    opener.open(a_url)

    # Install the opener.
    # Now all calls to urllib2.urlopen use our opener.
    urllib2.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`, `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

urllib2 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 6. 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 = urllib2.ProxyHandler({})
    >>> opener = urllib2.build_opener(proxy_support)
    >>> urllib2.install_opener(opener)

注解

Currently `urllib2` does not support fetching of `https` locations through a proxy. However, this can be enabled by extending urllib2 as shown in the recipe 7.

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. urllib2 uses the httplib 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 httplib or urllib2 levels. However, you can set the default timeout globally for all sockets using

    import socket
    import urllib2

    # timeout in seconds
    timeout = 10
    socket.setdefaulttimeout(timeout)

    # this call to urllib2.urlopen now uses the default timeout
    # we have set in the socket module
    req = urllib2.Request('http://www.voidspace.org.uk')
    response = urllib2.urlopen(req)

## 备注

这篇文档由 John Lee 审订。

1

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

2

  例如 Google。

3

  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.

4

  The user agent for MSIE 6 is ‘Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; .NET CLR 1.1.4322)’

5

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

6

  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 urllib2 picks up on. In order to test scripts with a localhost server, I have to prevent urllib2 from using the proxy.

7

  urllib2 opener for SSL proxy (CONNECT method): ASPN Cookbook Recipe.