Python基础【day03】：字典（一）

本节内容

1. 简述
2. 语法
3. 字典的使用
4. 字典内置方法

一、简述

字典是Python中的又一种数据结构，它是通过key-value的数据类型，跟我们小时候用的新华字典差不多，通过key去访问value

二、语法

stu_info = {
    "xiedi":28,
    "liuhailin":27,
    "daiqiao":30,
    "hanwenhai":25,
    "chenqun":38
}

2、特性

①字典是无序的

②字典是通过key去访问value（元素不存在重复）

>>> stu_info = { "xiedi":28, "liuhailin":27,"daiqiao":30,"hanwenhai":25,"chenqun":38}
#问通过key值访问对应的元素
>>> stu_info["chenqun"]
38
>>>

三、字典的使用

1、增加

>>> stu_info = { "xiedi":28, "liuhailin":27,"daiqiao":30,"hanwenhai":25,"chenqun":38}
>>> stu_info["chenqun"]
38
#新增
>>> stu_info["luoahong"]=32
>>> stu_info
{'hanwenhai': 25, 'daiqiao': 30, 'luoahong': 32, 'chenqun': 38, 'xiedi': 28, 'liuhailin': 27}

2、修改

>>> stu_info = { "xiedi":28, "liuhailin":27,"daiqiao":30,"hanwenhai":25,"chenqun":38}
#修改"xiedi"对应的value值，由28改为29
>>> stu_info["xiedi"]=29
#输出
>>> stu_info
{'hanwenhai': 25, 'daiqiao': 30, 'chenqun': 38, 'xiedi': 29, 'liuhailin': 27}

3、删除(del、pop、popitem)

del

>>> stu_info = { "xiedi":28, "liuhailin":27,"daiqiao":30,"hanwenhai":25,"chenqun":38}
>>> del stu_info["chenqun"]
#结果输出
>>> stu_info
{'hanwenhai': 25, 'daiqiao': 30, 'xiedi': 29, 'liuhailin': 27}

特别提醒：如果是del stu_info的话，则是删除stu_info这个变量

pop(k)

>>> stu_info = { "xiedi":28, "liuhailin":27,"daiqiao":30,"hanwenhai":25,"chenqun":38}
#删除时给出提示
>>> stu_info.pop("liuhailin")
27
#输出结果
>>> stu_info
{'hanwenhai': 25, 'daiqiao': 30, 'chenqun': 38, 'xiedi': 28}

popitem()

随机删除字典中的一个元素

>>> stu_info = { "xiedi":28, "liuhailin":27,"daiqiao":30,"hanwenhai":25,"chenqun":38}
#随机删除的元素
>>> stu_info.popitem()
('hanwenhai', 25)
#输出结果
>>> stu_info
{'daiqiao': 30, 'chenqun': 38, 'xiedi': 28, 'liuhailin': 27}

4、查找

key值在字典中存在

>>> stu_info = { "xiedi":28, "liuhailin":27,"daiqiao":30,"hanwenhai":25,"chenqun":38}
>>> "chenqun" in stu_info
True
>>> stu_info = { "xiedi":28, "liuhailin":27,"daiqiao":30,"hanwenhai":25,"chenqun":38}
>>> stu_info.has_key("daiqiao")　　##标准用法,在Python3和Python2.7都可以用
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'dict' object has no attribute 'has_key'

以上这种用法，在Python3和Python2.7都可以用，但是下面这种情况只能在Python2.7上用，Python3已经废除了

get(k)

>>> stu_info = { "xiedi":28, "liuhailin":27,"daiqiao":30,"hanwenhai":25,"chenqun":38}
#存在则返回对应的value
>>> print(stu_info.get("daiqiao"))
30
>>> print(stu_info.get("hanwenhai"))
25

通过k获取value

>>> stu_info = { "xiedi":28, "liuhailin":27,"daiqiao":30,"hanwenhai":25,"chenqun":38}
#存在key返回value
>>> stu_info["liuhailin"]
27
#不存在key提示报错
>>> stu_info["luoahong"]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'luoahong'

 小结：get(k)不存在key值，则返回None，通过key直接访问会报错，所以建议推荐用get(k)这个方法获取value

5、多级字典嵌套及操作

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

av_catalog = {     "欧美":{         "www.youporn.com": ["很多免费的,世界最大的","质量一般"],         "www.pornhub.com": ["很多免费的,也很大","质量比yourporn高点"],         "letmedothistoyou.com": ["多是自拍,高质量图片很多","资源不多,更新慢"],         "x-art.com":["质量很高,真的很高","全部收费,屌比请绕过"]     },     "日韩":{         "tokyo-hot":["质量怎样不清楚,个人已经不喜欢日韩范了","听说是收费的"]     },     "大陆":{         "1024":["全部免费,真好,好人一生平安","服务器在国外,慢"]     } }   av_catalog["大陆"]["1024"][1] += ",可以用爬虫爬下来" print(av_catalog["大陆"]["1024"]) #ouput ['全部免费,真好,好人一生平安', '服务器在国外,慢,可以用爬虫爬下来']

四、字典的内容之方法

1、values()

返回字典中所有value，生成一个列表

>>> stu_info = { "xiedi":28, "liuhailin":27,"daiqiao":30,"hanwenhai":25,"chenqun":38}
>>> stu_info.values()
#输出
dict_values([25, 30, 38, 28, 27])
>>>

2、keys()

返回字典中所有key，生成一个列表

>>> stu_info = { "xiedi":28, "liuhailin":27,"daiqiao":30,"hanwenhai":25,"chenqun":38}
>>> stu_info.keys()
#输出结果
dict_keys(['hanwenhai', 'daiqiao', 'chenqun', 'xiedi', 'liuhailin'])

3、setdefault(k,v)

setdefault()表示去取字典中的key，如果取不到，则设置新值，相反如果取到，则返回原有默认值

>>> stu_info = { "xiedi":28, "liuhailin":27,"daiqiao":30,"hanwenhai":25,"chenqun":38}
#设置不存在的key
>>> stu_info.setdefault("luoahong",32)
32
>>> stu_info
#输出结果
{'hanwenhai': 25, 'daiqiao': 30, 'luoahong': 32, 'chenqun': 38, 'xiedi': 28, 'liuhailin': 27}
#设置存在的key
>>> stu_info.setdefault("chenqun",28)
38
#输出结果
>>> stu_info
{'hanwenhai': 25, 'daiqiao': 30, 'luoahong': 32, 'chenqun': 38, 'xiedi': 28, 'liuhailin': 27}
>>>

4、update(dict)

update()是把两个字典合并成一个新的字典，中间有交叉的key，更新替换成新值，没有交叉就直接创建

>>> stu_info = { "xiedi":28, "liuhailin":27,"daiqiao":30,"hanwenhai":25,"chenqun":38}
>>> yhd={"luoahong":32,1:2,3:4}
>>> stu_info.update(yhd)
>>> stu_info
#输出
{1: 2, 3: 4, 'chenqun': 38, 'xiedi': 28, 'hanwenhai': 25, 'daiqiao': 30, 'luoahong': 32, 'liuhailin': 27}

注：以后提到更新(update),就是直接原有的字典或者列表上修改，添加(上面的例子就是在stu_info基础上更新的)

5、items()

把字典转换成列表

>>> stu_info = { "xiedi":28, "liuhailin":27,"daiqiao":30,"hanwenhai":25,"chenqun":38}
>>> stu_info.items()
dict_items([('hanwenhai', 25), ('daiqiao', 30), ('chenqun', 38), ('xiedi', 28), ('liuhailin', 27)])

6、fromkeys(list,默认值)

初始化一个字典

>>> dict.fromkeys(["xiedi","liuhailin"],1)
{'xiedi': 1, 'liuhailin': 1}

特别提醒:创建多层会有浅copy的作用，所以不建议使用，只需要做了解即可

7、clear()

清空字典

>>> stu_info = { "xiedi":28, "liuhailin":27,"daiqiao":30,"hanwenhai":25,"chenqun":38}
>>> stu_info.clear()
>>> stu_info
{}