effective python读书笔记——get处理不在字典中的情况

#处理键不在字典中的情况

#查询字典中是否有某键，有则键值+1，无则创建新键，值+1

#常规方法：in 或 KeyError

dictionary={'first':1,'second':2}

key1='third'

if key1 in dictionary:

    count1=dictionary[key1]

else:

    count1=0

dictionary[key1]=count1+1

#简化key1：

if key1 not in dictionary:

    dictionary[key1]=0

dictionary[key1]+=1

key2='forth'

try:

    count2=dictionary[key2]

except KeyError:

    count2=0

dictionary[key2]=count2+1

#简化key2

try:

    dictionary[key2]+=1

except KeyError:

    dictionary[key2]=1

#get方法 第一个参数为想查询的键，第二个参数是键不存在时返回的默认值

key3='fifth'

count3=dictionary.get(key3,0)

dictionary[key3]=count3+1

print(dictionary)

#如果字典保存的数据类型比较复杂，如list

dictionaries={'first':['a','A'],'second':['b','B']}

key1='third'

value1='c'

value2='C'

if key1 in dictionaries:

    list1=dictionaries[key1]

else:

    dictionaries[key1]=list1=[]#完成空白列表赋给list1，并与key1关联

list1.append(value1)

try:

    list2=dictionaries[key1]

except KeyError:

    dictionaries[key1]=list2=[]

list2.append(value2)

#用get改写

key2='forth'

value3='d'

value4='D'

list3=dictionaries.get(key2)

if list3 is None:

    dictionaries[key2]=list3=[]

list3.append(value3)

#加上海象运算符

if(list4:=dictionaries.get(key2)) is None:

    dictionaries[key2]=list4=[]

list4.append(value4)

#setdefault方法

#查询字典里是否有某键有则返回值，无则将提供的默认值此键关联并插入字典

key='wsz'

name=dictionaries.setdefault(key,[])

name.append('nb')

print(dictionaries)