dict的特性和基本语法——python3.6

• 特性
• key:value结构，字典中的每一个元素，都是键值对
• key必须可被hash，且必须为不可变数据类型，必须唯一
• 可存放任意多个值，可修改，可以不唯一
• 无序
• 查找速度快，因为hash可以把key变成数字，数字则可通过二分法快速查找，提高查找效率
• 基本语法
  • clear：清空字典

  • Info = {'stu1':'LiLei','stu2':'HanMeiMei','stu3':'XiaoMing'}
    print(Info)    # =>{'stu1': 'LiLei', 'stu2': 'HanMeiMei', 'stu3': 'XiaoMing'}
    Info.clear()
    print(Info)    # =>{}

  • copy：复制字典

  • Info = {'stu1':'LiLei','stu2':'HanMeiMei','stu3':'XiaoMing'}
    Info_new = Info.copy()
    print(Info_new)   # =>{'stu1': 'LiLei', 'stu2': 'HanMeiMei', 'stu3': 'XiaoMing'}

• • fromkeys：返回一个新的字典，key值可迭代，但所有的value都是相等的

  • Info = {}
    Info_new = Info.fromkeys([1,2,3],'Hi')
    print(Info_new)   # =>{1: 'Hi', 2: 'Hi', 3: 'Hi'}

  • get：通过key获取对应的value，如果字典中key存在，则返回对应的value，如果不存在，返回None

  • Info = {'stu1':'LiLei','stu2':'HanMeiMei','stu3':'XiaoMing'}
    print(Info.get('stu2'))# =>HanMeiMei
    print(Info.get('stu4'))# =>None

  • items：获取所有的key-value，以列表方式呈现

  • Info = {'stu1':'LiLei','stu2':'HanMeiMei','stu3':'XiaoMing'}
    print(Info.items())# =>dict_items([('stu1', 'LiLei'), ('stu2', 'HanMeiMei'), ('stu3', 'XiaoMing')])

  • keys：获取所有的key

  • Info = {'stu1':'LiLei','stu2':'HanMeiMei','stu3':'XiaoMing'}
    print(Info.keys()) # =>dict_keys(['stu1', 'stu2', 'stu3'])

  • values：获取所有的values

  • Info = {'stu1':'LiLei','stu2':'HanMeiMei','stu3':'XiaoMing'}
    print(Info.values()) # =>dict_values(['LiLei', 'HanMeiMei', 'XiaoMing'])

  • pop：pop(self,k,d=None)删除指定的key，如果key存在，则返回对应的value，如果key不存在，则返回指定的d，如果d未指定，则报错

  • Info = {'stu1':'LiLei','stu2':'HanMeiMei','stu3':'XiaoMing'}
    print(Info.pop('stu1'))# =>LiLei
    print(Info.pop('stu4',0))# =>0
    print(Info.pop('stu4'))# =>报错

  • popitem：删除字典中的一对key-value，无序的，当字典为空时，报错

  • Info = {'stu1':'LiLei','stu2':'HanMeiMei','stu3':'XiaoMing'}
    print(Info.popitem())# =>('stu3', 'XiaoMing')
    print(Info.popitem())# =>('stu2', 'HanMeiMei')
    print(Info.popitem())# =>('stu1', 'LiLei')
    print(Info.popitem())# =>报错

  • setdefault：setdefault(self,k,d=None)如果key存在，则返回对应的value，如果key不存在，则将该key-value添加进字典，并返回该value（即指定的d），若d未指定，则为None 

  • Info = {'stu1':'LiLei','stu2':'HanMeiMei','stu3':'XiaoMing'}
    print(Info.setdefault('stu3','Jack'))# =>XiaoMing
    print(Info)# =>{'stu1': 'LiLei', 'stu2': 'HanMeiMei', 'stu3': 'XiaoMing'}
    print(Info.setdefault('stu4','Jack'))# =>Jack
    print(Info)# =>{'stu1': 'LiLei', 'stu2': 'HanMeiMei', 'stu3': 'XiaoMing', 'stu4': 'Jack'}

  • update：更新字典，若key不重复，则类似增添新的key-value；若key重复，则key对应的value会被覆盖

  • Info = {'stu1':'LiLei','stu2':'HanMeiMei','stu3':'XiaoMing'}
    Info_new = {'stu4':'Jack','stu5':'Tony','stu6':'Tom'}
    Info.update(Info_new)
    print(Info)# =>{'stu1': 'LiLei', 'stu2': 'HanMeiMei', 'stu3': 'XiaoMing', 'stu4': 'Jack', 'stu5': 'Tony', 'stu6': 'Tom'}

    Info = {'stu1':'LiLei','stu2':'HanMeiMei','stu3':'XiaoMing'}
    Info_new = {'stu1':'Jack','stu5':'Tony','stu6':'Tom'}
    Info.update(Info_new)
    print(Info)# =>{'stu1': 'Jack', 'stu2': 'HanMeiMei', 'stu3': 'XiaoMing', 'stu5': 'Tony', 'stu6': 'Tom'}

• 其他语法
  • 循环输出所有的key

  • Info = {'stu1':'LiLei','stu2':'HanMeiMei','stu3':'XiaoMing'}
    for key in Info:
        print(key)

  • Info = {'stu1':'LiLei','stu2':'HanMeiMei','stu3':'XiaoMing'}
    for key in Info.keys():
        print(key)

  • 循环输出所有的value

  • Info = {'stu1':'LiLei','stu2':'HanMeiMei','stu3':'XiaoMing'}
    for value in Info.values():
        print(value)

  • 循环输出所有的key-value

  • Info = {'stu1':'LiLei','stu2':'HanMeiMei','stu3':'XiaoMing'}
    for key in Info:
        print(key,Info[key])

  • del：删除指定的key，若key不存在，则报错

  • Info = {'stu1':'LiLei','stu2':'HanMeiMei','stu3':'XiaoMing'}
    del Info['stu1']
    print(Info)# =>{'stu2': 'HanMeiMei', 'stu3': 'XiaoMing'}
    del Info['stu4']# =>报错