如何使用 urllib 套件取得網路資源¶

作者:: Michael Foord

簡介¶

urllib.request 是一個用來從 URLs (Uniform Resource Locators) 取得資料的Python模組。它提供一個了非常簡單的介面能接受多種不同的協定，urlopen 函式。也提供了較複雜的介面用於處理一些常見的狀況，例如：基本的 authentication、cookies、proxies 等等，這些都可以由 handler 或 opener 物件操作。

urllib.request 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.

一般情形下 urlopen 是非常容易使用的，但當你遇到錯誤或者較複雜的情況下，你可能需要對超文本協定 (HyperText Transfer Protocol) 有一定的了解。最完整且具參考價值的是 RFC 2616，不過它是一份技術文件並不容易閱讀，以下的教學會提供足夠的 HTTP 知識來幫助你使用 urllib。這份教學並非要取代 urllib.request 這份文件，你還是會需要它。

從 URL 取得資源¶

以下是使用 urllib.request 最簡單的方法:

import urllib.request
with urllib.request.urlopen('http://python.org/') as response:
   html = response.read()

If you wish to retrieve a resource via URL and store it in a temporary location, you can do so via the shutil.copyfileobj() and tempfile.NamedTemporaryFile() functions:

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

Many uses of urllib 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. urllib.request 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 urllib.request

req = urllib.request.Request('http://python.org/')
with urllib.request.urlopen(req) as response:
   the_page = response.read()

Note that urllib.request makes use of the same Request interface to handle all URL schemes. For example, you can make an FTP request like so:

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.

This is done as follows:

>>> 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.

Handling Exceptions¶

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.

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¶

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 that shows all the response codes used by RFC 2616. An excerpt from the dictionary is shown below

responses = {
    ...
    <HTTPStatus.OK: 200>: ('OK', 'Request fulfilled, document follows'),
    ...
    <HTTPStatus.FORBIDDEN: 403>: ('Forbidden',
                                  'Request forbidden -- authorization will '
                                  'not help'),
    <HTTPStatus.NOT_FOUND: 404>: ('Not Found',
                                  'Nothing matches the given URI'),
    ...
    <HTTPStatus.IM_A_TEAPOT: 418>: ("I'm a Teapot",
                                    'Server refuses to brew coffee because '
                                    