JSON file storage

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JSON File store

​ JSON, Its full name is Java Script Object Notation, That is to say JavaScript Object tag , Represents data through a combination of objects and arrays , Although the structure is simple, the degree of structure is very high , Is a lightweight data exchange format .

​ Now let's learn how to use Python Store data as JSON file .

1. Objects and arrays

​ stay JavaScript In language , Everything is the object , Therefore, any supported data type can pass JSON Express , For example, substring 、 Numbers 、 object 、 Array etc. . Among them, object and array are two special and commonly used types , Let's briefly introduce these two .

​ The object is JavaScript Middle means to use curly brackets {} Surrounded content , The data structure is {key1:value1,key2:value2,…} This key value pair structure . In an object-oriented language ,key Represents the properties of an object 、value Represents the value corresponding to the attribute , The former can be represented by integers and strings , The latter can be of any type .

​ Array in JavaScript Chinese means to use square brackets [] Surrounded content , The data structure is [“java”,“javascript”,“vb”,…】 This index structure . stay JavaScript in , Array is a special data type , Because it can also use key value pair structures like objects , But the index structure is used more . Again , Its value can be of any type .

​ therefore , One JSON Objects can be written in the following form ：

[{
    
    "name":"Bob",
    "gender":"male",
    "birthday":"1992-10-18"
},
{
    
    "name":"Selina",
    "gender":"female",
    "birthday":"1995-11-16"
}]

​ from [] The enclosed content is equivalent to an array , Each element in the array can be of any type , The elements in this instance are objects , from {} Surround .

​ JSON It can be freely combined from the above two forms , It can be nested infinite times , And the structure is clear , It is an excellent way to realize data exchange .

2. Read JSON

​ Python It provides us with a simple and easy to use JSON library , Used to implement JSON Read and write operation of the file , We can call JSON In the library loads Methods will JSON Text string to JSON object . actually ,JSON Object is Python Nesting and combination of lists and dictionaries in . In turn, , We can go through dumps Methods will JSON Object into a text string .

​ for example , Here's a passage JSON String of form , yes str type , We use it Python Turn it into an operational data structure , Like a list or dictionary ：

import json
str = ''' [{ "name":"Bob", "gender":"male", "birthday":"1992-10-18" }, { "name":"Selina", "gender":"female", "birthday":"1995-11-16" }] '''
print(type(str))
data = json.loads(str)
print(data)
print(type(data))

​ The operation results are as follows ：

​ Use here loads Method converts a string to JSON object . Because the outermost layer is brackets , So the final data type is list type .

​ thus , We can use the index to get the corresponding content . for example , To get the in the first element name attribute , You can use the following methods ：

import json
str = ''' [{ "name":"Bob", "gender":"male", "birthday":"1992-10-18" }, { "name":"Selina", "gender":"female", "birthday":"1995-11-16" }] '''
data = json.loads(str)
print(data[0]['name'])
print(data[0].get('name'))

The output is ：

​ Add... In brackets 0 As index , You can get the first dictionary element , Then call its key name to get the corresponding key value . There are two ways to get key values , One is to add key names in brackets , The other is to use get Method to pass in the key name . It is recommended to use get Method , In this way, even if the key name does not exist , No errors reported , But will return to None. in addition ,get Method can also pass in a second parameter （ Default value ）, Examples are as follows ：

import json
str = ''' [{ "name":"Bob", "gender":"male", "birthday":"1992-10-18" }, { "name":"Selina", "gender":"female", "birthday":"1995-11-16" }] '''
data = json.loads(str)
print(data[0].get('age'))
print(data[0].get('age', 25))

​ The operation results are as follows :

​ The second parameter , The value passed in will be returned . Here we try to get the age age, The key name does not exist in the original dictionary , Therefore, it will return by default None. At this time, if the second parameter is passed , The value passed in will be returned .

​ It is worth noting that ,JSON The data of needs to be enclosed in double quotation marks , Instead of using single quotes . For example, use the following form , There will be errors ：

import json
str = ''' [{ 'name':'Bob', 'gender':'male', 'birthday':'1992-10-18' }] '''
data = json.loads(str)

​ The operation results are as follows ：

json.decoder.JSONDecodeError: Expecting property name enclosed in double quotes: line 3 column 5 (char 8)

​ It's here JSON Tips for parsing errors , The reason is that the data is surrounded by single quotation marks . Again , Please pay attention to JSON Double quotation marks are required for string representation , otherwise loads Method will fail to parse . The following implementation is from JSON Read in text , For example, there is a data.json text file , Its content is just defined JSON word Fu string , We can read out the contents of the text file first , recycling loads Methods transform it into JSON object :

import json

with open('data.json', encoding='utf-8') as file:
    str = file.read()
    data = json.loads(str)
    print(data)

​ The operation results are as follows ：

​ Here we use open Method to read a text file , The default read mode is used , The encoding is specified as utf-8, And the file operation object is assigned file. Then we call file Object's read Method reads everything in the text , The assignment is str. Then call loads Method resolution JSON character string , Convert it to JSON object .

​ In fact, there is a simpler way to write the above examples , You can use it directly load Method passes in the file operation object , You can also convert text into JSON object , It is written as follows ：

import json

data = json. load(open('data.json', encoding='utf-8'))
print(data)

​ Pay attention to the use of load Method , instead of loads Method . The parameter of the former is a file operation object , The latter parameter is a JSON character string .

​ The operation results of these two methods are exactly the same . It's just load The method is to convert the contents of the entire file into JSON object , and loads Method can control which content to convert more flexibly . The two methods can be used in appropriate scenarios .

3. Output JSON

​ You can call dumps Methods will JSON Object to string . for example , Rewrite the list in the running result of the above example into the text ：

​ Here use dumps Method , take JSON Object to string , Then call the write Method writes a string to text , The results are shown in the following figure .

​ in addition , If you want to save JSON Indent format of object , Can go again dumps Method indent, Represents the number of indented characters . Examples are as follows ：

import json
data = [{
    
    'name': 'Bob',
    'gender': 'male',
    'birthday': '1992-10-18'
}]
with open('data.json', 'w', encoding='utf-8') as file:
    file.write(json.dumps(data, indent=2))

​ At this time, the write result is shown in the figure .

​ Can see , The content obtained has its own indentation , The format is clearer .

​ in addition , If JSON Object contains Chinese characters , What will happen ？ Now before JSON Some values in the object are changed to Chinese , And still use the previous method to write it into the text ：

import json
data = [{
    
    'name': ' Zhang San ',
    'gender': ' male ',
    'birthday': '1992-10-18'
}]
with open('data.json', 'w', encoding='utf-8') as file:
    file.write(json.dumps(data, indent=2))

The write result is shown in the figure ：

​ You can see , The Chinese characters in the text have become Unicode character , This is obviously not what we want .

​ To output Chinese , You also need to specify parameters ensure_ascii by False, And specify the code of document output ：

import json
data = [{
    
    'name': ' Zhang San ',
    'gender': ' male ',
    'birthday': '1992-10-18'
}]
with open('data.json', 'w', encoding='utf-8') as file:
    file.write(json.dumps(data, indent=2, ensure_ascii=False))

The write result at this time is shown in the figure ：

​ Can be found , Now we can put JSON Object output is Chinese .

​ analogy loads And load Method ,dumps There is also a corresponding dump Method , It can directly transfer JSON All objects are written to the file , Therefore, the above writing method can also be written in the following form ：

json.dump(data, open('data.json','w', encoding='utf-8'), indent=2, ensure_ascii=False)

​ The first parameter here is JSON object , The second parameter can be passed into the file operation object , Other indent、ensure_ascii The object remains unchanged , The results are the same .