1. 在學習廖雪峰老師的Python教程時，學到了難以理解的關於SAX解析XML這一節。本文將從此節出發，逐句的分析此節的作業。其中作業來源於網友評論。
2. SAX解析XML速度快、佔用記憶體小。我們只需要關注三個事件：start_element、end_element、char_data。如：當SAX在解析一個節點時<li><a href="/python">Python</a></li> 會產生三個事件：
   2.1 start_element事件，分別讀取<li>、<a href="/python">
   2.2 end_element
   事件，分別讀取</a>、</li>
   2.3 char_data事件、讀取Python
3. 補充二維字典知識：
   3.1 定義二維字典 ：dict_2d = {'a': {'a': 1, 'b': 3}, 'b': {'a': 6}}
   3.2 訪問二維字典：dict_2d['a']['a'] ,結果明顯是:1
   3.3 新增二維字典中屬性時可以用一個函式來完成：

#二維字典的新增函式
def addtwodimdict(thedict, key_a, key_b, val):
  if key_a in adic:
    thedict[key_a].update({key_b: val})
  else
 
:
    thedict.update({key_a:{key_b: val}})

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4. 例項：請利用SAX編寫程式解析Yahoo的XML格式的天氣預報，獲取當天和第二天的天氣：

#！/uer/bin/env python
#-*- coding:utf-8 -*-
#使用SAX解析XML
#查詢yahoo天氣的今天和明天天氣
#題目及程式碼來源：https://www.liaoxuefeng.com/wiki/0014316089557264a6b348958f449949df42a6d3a2e542c000/001432002075057b594f70ecb58445da6ef6071aca880af000
#宣告
from xml.parsers.expat import
 
 ParserCreate
#定義天氣字典、天數
weather_dict = {}
which_day = 0
#定義解析類
#包括三個主要函式：start_element(),end_element(),char_data()
class WeatherSaxHandler(object):
    #定義start_element函式
    def start_element(self,name,attrs):
        global weather_dict,which_day
        #判斷並獲取XML文件中地理位置資訊
        if name == 'yweather:location':
            #將本行XML程式碼中'city'屬性值賦予字典weather_dict中的'city'
            weather_dict['city']=attrs['city']
            weather_dict['country']=attrs['country']#執行結束後此時，weather_dict={'city':'Beijing','country'='China'}
        #同理獲取天氣預測資訊
        if name == 'yweather:forecast':
            which_day +=1
            #第一天天氣，獲取氣溫、天氣
            if which_day == 1:
                weather ={'text':attrs['text'],
                          'low':int(attrs['low']),
                          'high':int(attrs['high'])
                          }
                weather_dict['today']=weather#此時weather_dict出現二維字典
#weather_dict={'city': 'Beijing', 'country': 'China', 'today': {'text': 'Partly Cloudy', 'low': 20, 'high': 33}}
            #第二天相關資訊
            elif which_day==2:
                weather={
                    'text':attrs['text'],
                    'low':int(attrs['low']),
                    'high':int(attrs['high'])
                }
                weather_dict['tomorrow']=weather
#weather_dict={'city': 'Beijing', 'country': 'China', 'today': {'text': 'Partly Cloudy', 'low': 20, 'high': 33}, 'tomorrow': {'text': 'Sunny', 'low': 21, 'high': 34}}
    #end_element函式
    def end_element(self,name):
        pass
    #char_data函式
    def char_data(self,text):
        pass


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 weather_dict

#XML文件，輸出結果的資料來源
#將XML文件賦值給data

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>
'''
#例項化類
weather = parse_weather(data)
#檢查條件是否為True
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']
#列印到螢幕
print('Weather:', str(weather))

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5. 通過本節的學習，對Python中SAX對XML的解析有了簡單的瞭解，為以後爬蟲學習做準備。