Python 字典

1. 概述

字典是另一种可变容器模型，且可存储任意类型对象。

字典的每个键值(key=>value)对用冒号(:)分割，每个对之间用逗号(,)分割，整个字典包括在花括号({})中 ,格式如下所示：

dict = {"name":"Jentle","age":18,"height":182.5}
print(dict)#{'name': 'Jentle', 'age': 18, 'height': 182.5}

键必须是唯一的，但值则不必。

值可以取任何数据类型，但键必须是不可变的，如字符串，数字或元组。

一个简单的字典实例：

#键必须是不可变的，如字符串、数字或元组
tup1 = ("123",1,1.2)
dict = {"name":"Jentle",2:18,tup1:182.5}
print(dict[2])#18
print(dict[tup1])#182.5

如果用字典里没有的键访问数据，会输出错误如下：

dict = {"name":"Jentle","age":18,"height":182.5}
print(dict["weight"])

#output:
Traceback (most recent call last):
  File "D:/Python/PythonCode/002PythonBasic/day002/00PythonNote.py", line 3, in <module>
    print(dict["weight"])
KeyError: 'weight'

防止字典里没有键访问报错，可以用get方法

dict = {"name":"Jentle","age":18,"height":182.5}
print(dict.get("weight",None))#output:None

2. 修改字典

向字典添加新内容的方法是增加新的键/值对，修改或删除已有键/值对如下实例:

dict = {"name":"Jentle","age":18,"height":182.5}
dict["age"] = 19 #更新
dict["weight"] = 75 #添加
print ("dict['age']: ", dict['age'])
print ("dict['weight']: ", dict['weight'])

3. 删除字典元素

能删单一的元素也能清空字典，清空只需一项操作。

显式删除一个字典用del命令，如下实例：

dict = {"name":"Jentle","age":18,"height":182.5}
print(dict) #{'name': 'Jentle', 'age': 18, 'height': 182.5}
del dict["age"]
print(dict) #{'name': 'Jentle', 'height': 182.5}
dict.clear() #清空字典
print(dict) #{}
del  dict #删除字典

4. 字典键的特性

字典值可以是任何的 python 对象，既可以是标准的对象，也可以是用户定义的，但键不行。

两个重要的点需要记住：

• 1.不允许同一个键出现两次。创建时如果同一个键被赋值两次，后一个值会被记住，如下实例：

dict = {"name":"Jentle","age":18,"height":182.5,"age":19}
print(dict) #{'name': 'Jentle', 'age': 18, 'height': 182.5}

• 2.键必须不可变，所以可以用数字，字符串或元组充当，而用列表就不行，如下实例：

dict = {["name"]:"Jentle","age":18,"height":182.5,"age":19}
print("dict[]",dict["[name]"])

#output:

Traceback (most recent call last):
 File "D:/Python/PythonCode/002PythonBasic/day002/00PythonNote.py", line 2, in <module>
   dict = {["name"]:"Jentle","age":18,"height":182.5,"age":19}
 TypeError: unhashable type: 'list'

5. 字典内置函数&方法

dict = {"name":"Jentle","age":18,"height":182.5,"age":19}
print(len(dict)) # 3
dictStr = str(dict)
print(dictStr)
dictType = type(dict)
print(dictType)

#fromkeys
seq = ('Google', 'Runoob', 'Taobao')

dict = dict.fromkeys(seq)
print("dict.fromkeys(seq) : %s" % str(dict))#dict.fromkeys(seq) : {'Google': None, 'Runoob': None, 'Taobao': None}

dict = dict.fromkeys(seq, 10)
print("dict.fromkeys(seq, 10) : %s" % str(dict))#dict.fromkeys(seq, 10) : {'Google': 10, 'Runoob': 10, 'Taobao': 10}

#字典 get() 方法
dict = {"name":"Jentle","age":18,"height":182.5,"age":19}
value1 = dict.get("name","default")
value2 = dict.get("sex","未设置")
print("value1=",value1)#value1= Jentle
print("value2=",value2)#value2= 未设置

# 字典 items() 方法:以列表返回可遍历的(键, 值) 元组数组
dict = {"name":"Jentle","age":18,"height":182.5,"age":19}
v = dict.items()
print(type(v))#<class 'dict_items'>
print(v)#dict_items([('name', 'Jentle'), ('age', 19), ('height', 182.5)])
#遍历
for key,value in dict.items():
    print(key,":\t",value)
'''
name :   Jentle
age :    19
height :     182.5
'''
dict2 = {"weight":75,"address":"北京"}
dict.update(dict2)#方法没有任何返回值
print(dict)#{'name': 'Jentle', 'age': 19, 'height': 182.5, 'weight': 75, 'address': '北京'}

#删除字典给定键 key 所对应的值，返回值为被删除的值。
popObj = dict.pop("address")
print(popObj)#北京

6. 字典的遍历

字典支持for循环遍历，直接遍历字典得到的为字典的key

dict = {"a":"A","b":"B","c":"C"}
for i in dict:
    print(i)
#output:
a
b
c

当然也可以通过for循环遍历字典的items来遍历键值对

for key,value in dict.items():
    print(key,value)
#output:
a A
b B
c C