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

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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）

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

三、字典的使用

1、增加

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

 2、修改

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>>> 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<br>>>> stu_info["xiedi"]=29 #输出<br>>>> stu_info {'hanwenhai': 25, 'daiqiao': 30, 'chenqun': 38, 'xiedi': 29, 'liuhailin': 27}

 3、删除(del、pop、popitem)

 del

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

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

popitem()

 随机删除字典中的一个元素

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

 4、查找

  key值在字典中存在

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>>> 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)

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

  通过k获取value

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>>> stu_info = { "xiedi":28, "liuhailin":27,"daiqiao":30,"hanwenhai":25,"chenqun":38} #存在key返回value<br>>>> stu_info["liuhailin"] 27 #不存在key提示报错<br>>>> 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

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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

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

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1 2	>>> stu_info = { "xiedi":28, "liuhailin":27,"daiqiao":30,"hanwenhai":25,"chenqun":38}<br>>>> stu_info.keys()<br>#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

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

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>>> 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<br>#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

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>>> 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

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1 2	>>> 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

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