使用Python处理JSON数据
25.1 JSON简介
25.1.1 什么是JSON
JSON全称为JavaScript Object Notation，一般翻译为JS标记，是一种轻量级的数据交换格式。是基于ECMAScript的一个子集，采用完全独立于编程语言的文本格式来存储和表示数据。简洁和清晰的层次结构使得JSON成为理想的数据交换语言，其主要特点有：易于阅读、易于机器生成、有效提升网络速度等。
25.1.2 JSON的两种结构
JSON简单来说，可以理解为JavaScript中的数组和对象，通过这两种结构，可以表示各种复杂的结构。
25.1.2.1 数组
数组在JavaScript是使用中括号[ ]来定义的，一般定义格式如下所示：
let array=["Surpass","28","Shanghai"];
若要对数组取值，则需要使用索引。元素的类型可以是数字、字符串、数组和对象等。
25.1.2.2 对象
对象在JavaScript是使用大括号{ }来定义的，一般定义格式如下所示：
let personInfo={name:"Surpass",age:28,location:"Shanghai"
}
对象一般是基于key和value，在JavaScript中，其取值方式也非常简单variable.key即可。元素value的类型可以是数字、字符串、数组和对象等。
25.1.3 支持的数据格式
JSON支持的主要数据格式如下所示：
- 数组：使用中括号
- 对象：使用大括号
- 整型、浮点型、布尔类型和null
- 字符串类型：必须使用双引号，不能使用单引号
多个数据之间使用逗号做为分隔符，基与Python中的数据类型对应表如下所示：
| JSON | Python |
|---|---|
| Object | dict |
| array | list |
| string | str |
| number(int) | int |
| number(real) | float |
| true | True |
| false | False |
| null | None |
25.2 Python对JSON的支持
25.2.1 Python 和 JSON 数据类型
在Python中主要使用json模块来对JSON数据进行处理。在使用前，需要导入json模块，用法如下所示：
import json
json模块中主要包含以下四个操作函数，如下所示：
在json的处理过种中，Python中的原始类型与JSON类型会存在相互转换，具体的转换表如下所示：
- Python 转换为 JSON
| Python | JSON |
|---|---|
| dict | Object |
| list | array |
| tuple | array |
| str | string |
| int | number |
| float | number |
| True | true |
| False | false |
| None | null |
- JSON 转换为 Python
| JSON | Python |
|---|---|
| Object | dict |
| array | list |
| string | str |
| number(int) | int |
| number(real) | float |
| true | True |
| false | False |
| null | None |
25.2.2 json模块常用方法
关于Python 内置的json模块，可以查看之前我写的文章：https://www.cnblogs.com/surpassme/p/13034972.html
25.3 使用JSONPath处理JSON数据
内置的json模块，在处理简单的JSON数据时，易用且非常非常方便，但在处理比较复杂且特别大的JSON数据，还是有一些费力，今天我们使用一个第三方的工具来处理JSON数据，叫JSONPath。
25.3.1 什么是JSONPath
JSONPath是一种用于解析JSON数据的表达语言。经常用于解析和处理多层嵌套的JSON数据，其用法与解析XML数据的XPath表达式语言非常相似。
25.3.2 安装
安装方法如下所示：
# pip install -U jsonpath
25.3.3 JSONPath语法
JSONPath语法与XPath非常相似，其对应参照表如下所示：
| XPath | JSONPath | 描述 |
|---|---|---|
| / | $ | 根节点/元素 |
| . | @ | 当前节点/元素 |
| / | . or [] | 子元素 |
| .. | n/a | 父元素 |
| // | .. | 递归向下搜索子元素 |
| * | * | 通配符，表示所有元素 |
| @ | n/a | 访问属性，JSON结构的数据没有这种属性 |
| [] | [] | 子元素操作符(可以在里面做简单的迭代操作，如数据索引，根据内容选值等) |
| | | [,] | 支持迭代器中做多选 |
| n/a | [start :end :step] | 数组分割操作 |
| [] | ?() | 筛选表达式 |
| n/a | () | 支持表达式计算 |
| () | n/a | 分组，JSONPath不支持 |
以上内容可查阅官方文档：JSONPath - XPath for JSON
我们以下示例数据为例，来进行对比，如下所示：
{ "store": {"book": [ { "category": "reference","author": "Nigel Rees","title": "Sayings of the Century","price": 8.95},{ "category": "fiction","author": "Evelyn Waugh","title": "Sword of Honour","price": 12.99},{ "category": "fiction","author": "Herman Melville","title": "Moby Dick","isbn": "0-553-21311-3","price": 8.99},{ "category": "fiction","author": "J. R. R. Tolkien","title": "The Lord of the Rings","isbn": "0-395-19395-8","price": 22.99}],"bicycle": {"color": "red","price": 19.95}}
}
| XPath | JSONPath | 结果 |
|---|---|---|
| /store/book/author | $.store.book[*].author | 获取book节点中所有author |
| //author | $..author | 获取所有author |
| /store/* | $.store.* | 获取store的元素，包含book和bicycle |
| /store//price | $.store..price | 获取store中的所有price |
| //book[3] | $..book[2] | 获取第三本书所有信息 |
| //book[last()] | ..����[(@.�����ℎ−1)]..book[-1:] | 获取最后一本书的信息 |
| //book[position()❤️] | ..����[0,1]..book[:2] | 获取前面的两本书 |
| //book[isbn] | $..book[?(@.isbn)] | 根据isbn进行过滤 |
| //book[price<10] | $..book[?(@.price<10)] | 根据price进行筛选 |
| //* | $..* | 所有元素 |
在XPath中，下标是1开始，而在JSONPath中是从0开始
JSONPath在线练习网址：JSONPath Online Evaluator
25.3.4 JSONPath用法
其基本用法形式如下所示：
jsonPath(obj, expr [, args])
基参数如下所示：
- obj (object|array):
JSON数据对象
- expr (string):
JSONPath表达式
- args (object|undefined):
改变输出格式，比如是输出是值还是路径，
args.resultType可选的输出格式为："VALUE"、"PATH"、"IPATH"
- 返回类型为(array|false):
若返回array，则代表成功匹配到数据，false则代表未匹配到数据。
25.3.5 在Python中的使用
from jsonpath import jsonpath
import jsondata = {"store":{"book": [{"category": "reference","author": "Nigel Rees","title": "Sayings of the Century","price": 8.95},{"category": "fiction","author": "Evelyn Waugh","title": "Sword of Honour","price": 12.99},{"category": "fiction","author": "Herman Melville","title": "Moby Dick","isbn": "0-553-21311-3","price": 8.99},{"category": "fiction","author": "J. R. R. Tolkien","title": "The Lord of the Rings","isbn": "0-395-19395-8","price": 22.99}],"bicycle": {"color": "red","price": 19.95}}
}# 获取book节点中所有author
getAllBookAuthor=jsonpath(data,"$.store.book[*].author")
print(f"getAllBookAuthor is :{json.dumps(getAllBookAuthor,indent=4)}")
# 获取book节点中所有author
getAllAuthor=jsonpath(data,"$..author")
print(f"getAllAuthor is {json.dumps(getAllAuthor,indent=4)}")
# 获取store的元素，包含book和bicycle
getAllStoreElement=jsonpath(data,"$.store.*")
print(f"getAllStoreElement is {json.dumps(getAllStoreElement,indent=4)}")
# 获取store中的所有price
getAllStorePriceA=jsonpath(data,"$[store]..price")
getAllStorePriceB=jsonpath(data,"$.store..price")
print(f"getAllStorePrictA is {getAllStorePriceA}\ngetAllStorePriceB is {getAllStorePriceB}")
# 获取第三本书所有信息
getThirdBookInfo=jsonpath(data,"$..book[2]")
print(f"getThirdBookInfo is {json.dumps(getThirdBookInfo,indent=4)}")
# 获取最后一本书的信息
getLastBookInfo=jsonpath(data,"$..book[-1:]")
print(f"getLastBookInfo is {json.dumps(getLastBookInfo,indent=4)}")
# 获取前面的两本书
getFirstAndSecondBookInfo=jsonpath(data,"$..book[:2]")
print(f"getFirstAndSecondBookInfo is {json.dumps(getFirstAndSecondBookInfo,indent=4)}")
# 根据isbn进行过滤
getWithFilterISBN=jsonpath(data,"$..book[?(@.isbn)]")
print(f"getWithFilterISBN is {json.dumps(getWithFilterISBN,indent=4)}")
# 根据price进行筛选
getWithFilterPrice=jsonpath(data,"$..book[?(@.price<10)]")
print(f"getWithFilterPrice is {json.dumps(getWithFilterPrice,indent=4)}")
# 所有元素
getAllElement=jsonpath(data,"$..*")
print(f"getAllElement is {json.dumps(getAllElement,indent=4)}")
# 未能匹配到元素时
noMatchElement=jsonpath(data,"$..surpass")
print(f"noMatchElement is {noMatchElement}")
# 调整输出格式
controlleOutput=jsonpath(data,expr="$..author",result_type="PATH")
print(f"controlleOutput is {json.dumps(controlleOutput,indent=4)}")
最终输出结果如下扬尘：
getAllBookAuthor is :["Nigel Rees","Evelyn Waugh","Herman Melville","J. R. R. Tolkien"
]
getAllAuthor is ["Nigel Rees","Evelyn Waugh","Herman Melville","J. R. R. Tolkien"
]
getAllStoreElement is [[{"category": "reference","author": "Nigel Rees","title": "Sayings of the Century","price": 8.95},{"category": "fiction","author": "Evelyn Waugh","title": "Sword of Honour","price": 12.99},{"category": "fiction","author": "Herman Melville","title": "Moby Dick","isbn": "0-553-21311-3","price": 8.99},{"category": "fiction","author": "J. R. R. Tolkien","title": "The Lord of the Rings","isbn": "0-395-19395-8","price": 22.99}],{"color": "red","price": 19.95}
]
getAllStorePrictA is [8.95, 12.99, 8.99, 22.99, 19.95]
getAllStorePriceB is [8.95, 12.99, 8.99, 22.99, 19.95]
getThirdBookInfo is [{"category": "fiction","author": "Herman Melville","title": "Moby Dick","isbn": "0-553-21311-3","price": 8.99}
]
getLastBookInfo is [{"category": "fiction","author": "J. R. R. Tolkien","title": "The Lord of the Rings","isbn": "0-395-19395-8","price": 22.99}
]
getFirstAndSecondBookInfo is [{"category": "reference","author": "Nigel Rees","title": "Sayings of the Century","price": 8.95},{"category": "fiction","author": "Evelyn Waugh","title": "Sword of Honour","price": 12.99}
]
getWithFilterISBN is [{"category": "fiction","author": "Herman Melville","title": "Moby Dick","isbn": "0-553-21311-3","price": 8.99},{"category": "fiction","author": "J. R. R. Tolkien","title": "The Lord of the Rings","isbn": "0-395-19395-8","price": 22.99}
]
getWithFilterPrice is [{"category": "reference","author": "Nigel Rees","title": "Sayings of the Century","price": 8.95},{"category": "fiction","author": "Herman Melville","title": "Moby Dick","isbn": "0-553-21311-3","price": 8.99}
]
getAllElement is [{"book": [{"category": "reference","author": "Nigel Rees","title": "Sayings of the Century","price": 8.95},{"category": "fiction","author": "Evelyn Waugh","title": "Sword of Honour","price": 12.99},{"category": "fiction","author": "Herman Melville","title": "Moby Dick","isbn": "0-553-21311-3","price": 8.99},{"category": "fiction","author": "J. R. R. Tolkien","title": "The Lord of the Rings","isbn": "0-395-19395-8","price": 22.99}],"bicycle": {"color": "red","price": 19.95}},[{"category": "reference","author": "Nigel Rees","title": "Sayings of the Century","price": 8.95},{"category": "fiction","author": "Evelyn Waugh","title": "Sword of Honour","price": 12.99},{"category": "fiction","author": "Herman Melville","title": "Moby Dick","isbn": "0-553-21311-3","price": 8.99},{"category": "fiction","author": "J. R. R. Tolkien","title": "The Lord of the Rings","isbn": "0-395-19395-8","price": 22.99}],{"color": "red","price": 19.95},{"category": "reference","author": "Nigel Rees","title": "Sayings of the Century","price": 8.95},{"category": "fiction","author": "Evelyn Waugh","title": "Sword of Honour","price": 12.99},{"category": "fiction","author": "Herman Melville","title": "Moby Dick","isbn": "0-553-21311-3","price": 8.99},{"category": "fiction","author": "J. R. R. Tolkien","title": "The Lord of the Rings","isbn": "0-395-19395-8","price": 22.99},"reference","Nigel Rees","Sayings of the Century",8.95,"fiction","Evelyn Waugh","Sword of Honour",12.99,"fiction","Herman Melville","Moby Dick","0-553-21311-3",8.99,"fiction","J. R. R. Tolkien","The Lord of the Rings","0-395-19395-8",22.99,"red",19.95
]
noMatchElement is False
controlleOutput is ["$['store']['book'][0]['author']","$['store']['book'][1]['author']","$['store']['book'][2]['author']","$['store']['book'][3]['author']"
]
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