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08_Use python built-in json to implement local persistence of data

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Use python built-in json to implement local persistence of data

Four json functions

function
() Read the local json data file and return a string in JSON format read from the file object in a list form and deserialize it to PythonObject
() Deserialize JSON-formatted strings to PythonObject
() Put PythonObjectSerializes a string in JSON format and writes it to a file object.
() Put PythonObjectSerialized to JSON format strings

Save data locally as json files

use()

Instance data

class Person:
     name = None
     age = None
     def __init__(self, name, age):
          = name
          = age

 person1 = Person("Alice", 30)
 person2 = Person("Bob", 25)
 person3 = Person("Charlie", 35)
 person4 = Person("David", 28)
 person5 = Person("Eve", 32)

 persons = [person1, person2, person3, person4, person5]
 person_list = [person.__dict__ for person in persons]
 print(person_list)

 '''
 [{'name': 'Alice', 'age': 30}, {'name': 'Bob', 'age': 25}, {'name': 'Charlie', 'age': 35}, {'name': 'David', 'age': 28}, {'name': 'Eve', 'age': 32}]
 <class 'list'>

 Returns a list
 '''

object.__dict__

object.__dict__Is a special property in Python that stores instance properties of an object. Each Python object has one__dict__Properties, it is a dictionary that contains all instance properties of an object and their corresponding values

Save the object's data to json, you need to use it firstObject.__dict__Take out the dictionary form of each object (using list comprehension), save the dictionary form of each object in the list, and save the list in the json file

data_list = [person.__dict__ for person in persons] # List comprehension

  1. Save data

    def load_data():
    with open(data_file, 'r', encoding='utf-8') as file:
        return (file)

  2. Read data

    def save_data(persons):
    with open(data_file, 'w', encoding='utf-8') as file:
        ([person.__dict__ for person in persons], file, indent=2)

Things to note

The open() function automatically creates a file:

  1. In 'r' mode (read-only mode), if the file does not exist, FileNotFoundError will be directly thrown, and the file will not be created automatically.

  2. It will be automatically created only if the file does not exist when using 'w' (write mode) or 'a' (append mode).

    Actual case:

    class HouseService:
     house_list = []
     data_file = ((__file__), 'house_data.json')
     def __init__(self):
         self.load_data()
         if not self.house_list:
             house1 = House('1', 'lihua', '123456', 'Zhengzhou Zhongyuan District', 800, 'Not for rent')
             house2 = House('2', 'jack', '123452', 'Erqi District, Zhengzhou City', 900, 'Not for rent')
             self.house_list.append(house1)
             self.house_list.append(house2)
             self.save_data()

    
     # Load house data
     def load_data(self):
         try:
             with open(self.data_file, 'r', encoding='utf-8') as file:
                 data = (file)
                 self.house_list = [House(**house) for house in data]
         except FileNotFoundError:
             self.house_list = []
    
     # Save house data
     def save_data(self):
         with open(self.data_file, 'w', encoding='utf-8') as file:
             ([house.__dict__ for house in self.house_list], file, ensure_ascii=False, indent=4)

    In this case,load_data()If the function does not use exception capture, and there is nohouse_data.jsonWhen the file is filed, the system will directly throw an exception. After catching the exception, the data list will be initialized. The program can continue to proceed downwards.save_data()Save data and automatically create json files

    Therefore, during the development process, when writing and saving house data, pay attention to exception capture

Add data

If there is new data that needs to be saved, it cannot be used directlymode='a'， 'a'Append writes to the pattern will cause the JSON file to becomeMultiple independent objectsInstead of valid arrays, the newly added data will be saved directly outside []

[{...}, {...}] // Raw data
 {...} // New data appended (format error)

Therefore, to append data to the json file, you need to use the following method:

  1. Use the mode of read → modify → overwrite write (rather than append directly)

  2. Use the 'w' pattern to ensure that the complete JSON array structure is written every time

    import os
 import json

 # Sample data
 data = {
     "name": "Alice",
     "age": 30,
     "skills": ["Python", "Docker"],
     "is_active": True
 }

 data_dile = ((__file__), 'person_data.json')

 def save_data(data):
     with open(data_dile, 'w', encoding='utf-8') as file:
         (data, file, indent=2)

 def load_data():
     with open(data_dile, 'r', encoding='utf-8') as file:
         data = (file)
         return data

 exist_data = load_data() # Get existing data
 print(type(exist_data)) # <class 'list'>
 exist_data.append(data) # Want to append data to the list
 save_data(exist_data) # Save the list after appended data
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