python学习04——列表的操作

笨办法学python第38节

如何创建列表在第32节，形式如下:

本节主要是讲对列表的操作，首先讲了 mystuff.append('hello') 的工作原理，我的理解是，首先Python找到mystuff这个变量，然后进行append（）这个函数操作。其中需要注意的是括号（）里面有一个额外参数就是mystuff本身。

本文练习：

 # create a mapping of state to abbreviation
 states = {
     'Oregon': 'OR',
     'Florida': 'FL',
     'California': 'CA',
     'Michigan': 'MI'
 }

 # create a basic set of states and some cities in them
 cities = {
     'CA': 'San Francisco',
     'MI': 'Detroit',
     'FL': 'Jacksonville'
 }

 # add some more cities
 cities['NY'] = 'New York'
 cities['OR'] = 'Portland'

 # print out some cities
 print '-' * 10
 print "NY State has: ", cities['NY']
 print "OR State has: ", cities['OR']

 # print some states
 print '-'*10
 print "Michigan's abbreviation is: ", states['Michigan']
 print "Florida's abbreviation is: ", states['Florida']

 # do it by using the state then cities dict
 print '-'*10
 print "Michigan has: ", cities[states['Michigan']]
 print "Florida has: ", cities[states['Florida']]

 # print every state abbreviation
 print '-'*10
 for state, abbrev in states.items():
     print "%s is abbreviated %s" % (state, abbrev)

 # print every city in state   (why not sequence)
 print '-'*10
 for abbrev, city in cities.items():
     print "%s has the city %s" % (abbrev, city)

 # now do both at the same time
 print '-'*10
 for state, abbrev in states.items():
     print "%s state is abbreviated %s and has city %s" % (
         state, abbrev, cities[abbrev])

 print '-'*10
 # safely get a abbreviation by state that might not be there
 state = states.get('Texas', None)

 if not state:
     print "Sorry, no Texas."

 # get a city with a default value
 city = cities.get('TX', 'Does Not Exist')
 print "The city for the state 'TX' is: %s" % city

存在的问题：

1. 40行的输出城市名称，运行时输出的并不是顺序输出的，这个输出遵循的规律是什么？

2. states.items(),按照书里说的（）里面有一个额外参数states，所以在这个（）里面不加参数，因为里面的参数就是前面的列表，那如果想再加一个参数在（）里面，如何加？

回答：

1. Python内置了字典：dict的支持，dict全称dictionary，在其他语言中也称为map，使用键-值（key-value）存储，具有极快的查找速度。

dict内部存放的顺序和key放入的顺序是没有关系的，dict的查找是根据哈希表查找的，所以输出不是顺序输出的。

2. 这个和书里的那个函数不一样，书里的那个函数是append（），这个item（）相当于遍历这个列表，所以后面不再加参数。