DOM vs SAX
操作XML有两种方法：DOM和SAX。DOM会把整个XML读入内存，解析为树，因此占用内存大，解析慢，优点是可以任意遍历树的节点。SAX是流模式，边读边解析，占用内存小，解析快，缺点是我们需要自己处理事件。
正常情况下，优先考虑SAX，因为DOM实在太占内存。
在Python中使用SAX解析XML非常简洁，通常我们关心的事件是start_element，end_element和char_data，准备好这3个函数，然后就可以解析xml了。
举个例子，当SAX解析器读到一个节点时：
<a href="/">python</a>
会产生3个事件：
start_element事件，在读取<a href="/">时；
char_data事件，在读取python时；
end_element事件，在读取</a>时。
用代码实验一下：
from xml.parsers.expat import ParserCreate
class DefaultSaxHandler(object):
def start_element(self, name, attrs):
print('sax:start_element: %s, attrs: %s' % (name, str(attrs)))
def end_element(self, name):
print('sax:end_element: %s' % name)
def char_data(self, text):
print('sax:char_data: %s' % text)
xml = r'''<?xml version="1.0"?>
<ol>
<li><a href="/python">Python</a></li>
<li><a href="/ruby">Ruby</a></li>
</ol>
'''
handler = DefaultSaxHandler()
parser = ParserCreate()
parser.StartElementHandler = handler.start_element
parser.EndElementHandler = handler.end_element
parser.CharacterDataHandler = handler.char_data
parser.Parse(xml)
需要注意的是读取一大段字符串时，CharacterDataHandler可能被多次调用，所以需要自己保存起来，在EndElementHandler里面再合并。
除了解析XML外，如何生成XML呢？99%的情况下需要生成的XML结构都是非常简单的，因此，最简单也是最有效的生成XML的方法是拼接字符串：
L = []
L.append(r'<?xml version="1.0"?>')
L.append(r'<root>')
L.append('some & data')
L.append(r'</root>')
return ''.join(L)
<?xml version="1.0"?><root>some & data</root>
练习
请利用SAX编写程序解析Yahoo的XML格式的天气预报，获取当天和第二天的天气：
http://weather.yahooapis.com/forecastrss?u=c&w=2151330
参数w是城市代码，要查询某个城市代码，可以在weather.yahoo.com搜索城市，浏览器地址栏的URL就包含城市代码。
# -*- coding:utf-8 -*-
#请利用SAX编写程序解析Yahoo的XML格式的天气预报，获取当天和第二天的天气
from xml.parsers.expat import ParserCreate
class WeatherSaxHandler(object):
def __init__(self):
self.data = {}
self.isPubDate = False
def start_element(self, name, attrs):
if name == 'yweather:location':
self.data['city'] = attrs['city']
self.data['country'] = attrs['country']
elif name == 'pubDate':
self.isPubDate = True
elif name == 'yweather:forecast':
if attrs['day'] == self.today:
self.initDayInfo(attrs, 'today')
elif attrs['day'] == self.tomorrow:
self.initDayInfo(attrs, 'tomorrow')
def end_element(self, name):
pass
def char_data(self, text):
if self.isPubDate:
self.isPubDate = False
self.today = text.split(',')[0]
self.tomorrow = self.getTomorrow(self.today)
def initDayInfo(self, attrs, dayName):
self.data[dayName] = {}
self.data[dayName]['text'] = attrs['text']
self.data[dayName]['low'] = int(attrs['low'])
self.data[dayName]['high'] = int(attrs['high'])
def getTomorrow(self, dayOfWeek):
if dayOfWeek == 'Sun':
return 'Mon'
if dayOfWeek == 'Mon':
return 'Tue'
if dayOfWeek == 'Tue':
return 'Wed'
if dayOfWeek == 'Wed':
return 'Thu'
if dayOfWeek == 'Thu':
return 'Fri'
if dayOfWeek == 'Fri':
return 'Sat'
if dayOfWeek == 'Sat':
return 'Sun'
def parse_weather(xml):
handler = WeatherSaxHandler()
parser = ParserCreate()
parser.StartElementHandler = handler.start_element
parser.EndElementHandler = handler.end_element
parser.CharacterDataHandler = handler.char_data
parser.Parse(xml)
return handler.data
# 测试:
data = r'''<?xml version="1.0" encoding="UTF-8" standalone="yes" ?>
<rss version="2.0" xmlns:yweather="http://xml.weather.yahoo.com/ns/rss/1.0" xmlns:geo="http://www.w3.org/2003/01/geo/wgs84_pos#">
<channel>
<title>Yahoo! Weather - Beijing, CN</title>
<lastBuildDate>Wed, 27 May 2015 11:00 am CST</lastBuildDate>
<yweather:location city="Beijing" region="" country="China"/>
<yweather:units temperature="C" distance="km" pressure="mb" speed="km/h"/>
<yweather:wind chill="28" direction="180" speed="14.48" />
<yweather:atmosphere humidity="53" visibility="2.61" pressure="1006.1" rising="0" />
<yweather:astronomy sunrise="4:51 am" sunset="7:32 pm"/>
<item>
<geo:lat>39.91</geo:lat>
<geo:long>116.39</geo:long>
<pubDate>Wed, 27 May 2015 11:00 am CST</pubDate>
<yweather:condition text="Haze" code="21" temp="28" date="Wed, 27 May 2015 11:00 am CST" />
<yweather:forecast day="Wed" date="27 May 2015" low="20" high="33" text="Partly Cloudy" code="30" />
<yweather:forecast day="Thu" date="28 May 2015" low="21" high="34" text="Sunny" code="32" />
<yweather:forecast day="Fri" date="29 May 2015" low="18" high="25" text="AM Showers" code="39" />
<yweather:forecast day="Sat" date="30 May 2015" low="18" high="32" text="Sunny" code="32" />
<yweather:forecast day="Sun" date="31 May 2015" low="20" high="37" text="Sunny" code="32" />
</item>
</channel>
</rss>
'''
weather = parse_weather(data)
assert weather['city'] == 'Beijing', weather['city']
assert weather['country'] == 'China', weather['country']
assert weather['today']['text'] == 'Partly Cloudy', weather['today']['text']
assert weather['today']['low'] == 20, weather['today']['low']
assert weather['today']['high'] == 33, weather['today']['high']
assert weather['tomorrow']['text'] == 'Sunny', weather['tomorrow']['text']
assert weather['tomorrow']['low'] == 21, weather['tomorrow']['low']
assert weather['tomorrow']['high'] == 34, weather['tomorrow']['high']
print('Weather:', str(weather))
Weather: {'tomorrow': {'text': 'Sunny', 'high': 34, 'low': 21}, 'today': {'text': 'Partly Cloudy', 'high': 33, 'low': 20}, 'city': 'Beijing', 'country': 'China'}