在讲解 如何下载 web 页 和 如何从 URL 解析 XML时, 你已经学习了关于 HTML 处理 和 XML 处理， 接下来让我们来更全面地探讨有关 HTTP web 服务的主题。

• Amazon API 允许您从 Amazon.com 在线商店获取产品信息。
• National Weather Service (美国) 和 Hong Kong Observatory (香港) 通过 web 服务提供天气警报。
• Atom API 用来管理基于 web 的内容。
• Syndicated feeds 应用于 weblogs 和 新闻站点中带给您来自众多站点的最新消息。

例 11.1. openanything.py

如果您还没有下载本书附带的例子程序, 可以 下载本程序和其他例子程序。

import urllib2, urlparse, gzip
from StringIO import StringIO

USER_AGENT = 'OpenAnything/1.0 +http://diveintopython.org/http_web_services/'

class SmartRedirectHandler(urllib2.HTTPRedirectHandler):    
    def http_error_301(self, req, fp, code, msg, headers):  
        result = urllib2.HTTPRedirectHandler.http_error_301(
            self, req, fp, code, msg, headers)              
        result.status = code                                
        return result                                       

    def http_error_302(self, req, fp, code, msg, headers):  
        result = urllib2.HTTPRedirectHandler.http_error_302(
            self, req, fp, code, msg, headers)              
        result.status = code                                
        return result                                       

class DefaultErrorHandler(urllib2.HTTPDefaultErrorHandler):   
    def http_error_default(self, req, fp, code, msg, headers):
        result = urllib2.HTTPError(                           
            req.get_full_url(), code, msg, headers, fp)       
        result.status = code                                  
        return result                                         

def openAnything(source, etag=None, lastmodified=None, agent=USER_AGENT):
    '''URL, filename, or string --> stream

    This function lets you define parsers that take any input source
    (URL, pathname to local or network file, or actual data as a string)
    and deal with it in a uniform manner.  Returned object is guaranteed
    to have all the basic stdio read methods (read, readline, readlines).
    Just .close() the object when you're done with it.

    If the etag argument is supplied, it will be used as the value of an
    If-None-Match request header.

    If the lastmodified argument is supplied, it must be a formatted
    date/time string in GMT (as returned in the Last-Modified header of
    a previous request).  The formatted date/time will be used
    as the value of an If-Modified-Since request header.

    If the agent argument is supplied, it will be used as the value of a
    User-Agent request header.
    '''

    if hasattr(source, 'read'):
        return source

    if source == '-':
        return sys.stdin

    if urlparse.urlparse(source)[0] == 'http':                                      
        # open URL with urllib2                                                     
        request = urllib2.Request(source)                                           
        request.add_header('User-Agent', agent)                                     
        if etag:                                                                    
            request.add_header('If-None-Match', etag)                               
        if lastmodified:                                                            
            request.add_header('If-Modified-Since', lastmodified)                   
        request.add_header('Accept-encoding', 'gzip')                               
        opener = urllib2.build_opener(SmartRedirectHandler(), DefaultErrorHandler())
        return opener.open(request)                                                 
    
    # try to open with native open function (if source is a filename)
    try:
        return open(source)
    except (IOError, OSError):
        pass

    # treat source as string
    return StringIO(str(source))

def fetch(source, etag=None, last_modified=None, agent=USER_AGENT):  
    '''Fetch data and metadata from a URL, file, stream, or string'''
    result = {}                                                      
    f = openAnything(source, etag, last_modified, agent)             
    result['data'] = f.read()                                        
    if hasattr(f, 'headers'):                                        
        # save ETag, if the server sent one                          
        result['etag'] = f.headers.get('ETag')                       
        # save Last-Modified header, if the server sent one          
        result['lastmodified'] = f.headers.get('Last-Modified')      
        if f.headers.get('content-encoding', '') == 'gzip':          
            # data came back gzip-compressed, decompress it          
            result['data'] = gzip.GzipFile(fileobj=StringIO(result['data']])).read()
    if hasattr(f, 'url'):                                            
        result['url'] = f.url                                        
        result['status'] = 200                                       
    if hasattr(f, 'status'):                                         
        result['status'] = f.status                                  
    f.close()                                                        
    return result