9.5 Serialization with JSON

  • JSON (JavaScript Object Notation) is a text-based, human-and-computer-readable, data-interchange format used to represent objects as collections of name–value pairs.
  • Preferred data format for transmitting objects across platforms.

JSON Data Format

  • Similar to Python dictionaries
  • Each JSON object contains a comma-separated list of property names and values, in curly braces. 
    {"account": 100, "name": "Jones", "balance": 24.98}

  • JSON arrays, like Python lists, are comma-separated values in square brackets.

    [100, 200, 300]

  • Values in JSON objects and arrays can be:

    • strings in double quotes
    • numbers
    • JSON Boolean values true or false
    • null (like None in Python)
    • arrays
    • other JSON objects

Python Standard Library Module json

  • json module enables you to convert objects to JSON (JavaScript Object Notation) text format
  • Known as serializing the data
  • Following dictionary, which contains one key–value pair consisting of the key 'accounts' with its associated value being a list of dictionaries representing two accounts
In [10]:
accounts_dict = {'accounts': [
    {'account': 100, 'name': 'Jones', 'balance': 24.98},
    {'account': 200, 'name': 'Doe', 'balance': 345.67}]}

Serializing an Object to JSON

  • Write JSON to a file
  • json module’s dump function serializes the dictionary accounts_dict into the file
In [11]:
import json
In [12]:
with open('accounts.json', 'w') as accounts:
    json.dump(accounts_dict, accounts)
  • Resulting file contains the following text—reformatted slightly for readability:
    {"accounts": 
[{"account": 100, "name": "Jones", "balance": 24.98}, 
 {"account": 200, "name": "Doe", "balance": 345.67}]}
  • JSON delimits strings with double-quote characters.

Deserializing the JSON Text

  • json module’s load function reads entire JSON contents of its file object argument and converts the JSON into a Python object
  • Known as deserializing the data
In [13]:
with open('accounts.json', 'r') as accounts:
    accounts_json = json.load(accounts)
In [14]:
accounts_json
Out[14]:
{'accounts': [{'account': 100, 'name': 'Jones', 'balance': 24.98},
  {'account': 200, 'name': 'Doe', 'balance': 345.67}]}
In [15]:
accounts_json['accounts']
Out[15]:
[{'account': 100, 'name': 'Jones', 'balance': 24.98},
 {'account': 200, 'name': 'Doe', 'balance': 345.67}]
In [16]:
accounts_json['accounts'][0]
Out[16]:
{'account': 100, 'name': 'Jones', 'balance': 24.98}
In [17]:
accounts_json['accounts'][1]
Out[17]:
{'account': 200, 'name': 'Doe', 'balance': 345.67}

Displaying the JSON Text

  • json module’s dumps function (dumps is short for “dump string”) returns a Python string representation of an object in JSON format
  • Canbe used to display JSON in a nicely indented format
    • sometimes called “pretty printing”
  • When call includes the indent keyword argument, the string contains newline characters and indentation for pretty printing
    • Also can use indent with the dump function when writing to a file
In [18]:
with open('accounts.json', 'r') as accounts:
    print(json.dumps(json.load(accounts), indent=4))
    

{
    "accounts": [
        {
            "account": 100,
            "name": "Jones",
            "balance": 24.98
        },
        {
            "account": 200,
            "name": "Doe",
            "balance": 345.67
        }
    ]
}

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