Python - JSON

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JSON in Python

JSON in Python is a popular data format used for data exchange between systems. The json module provides functions to work with JSON data, allowing you to serialize Python objects into JSON strings and deserialize JSON strings back into Python objects.

JSON (JavaScript Object Notation) is a lightweight data interchange format that is easy for humans to read and write, and easy for machines to parse and generate. It is mainly used to transmit data between a server and web application as text.

JSON Serialization

JSON serialization is the process of converting a Python object into a JSON format. This is useful for saving data in a format that can be easily transmitted or stored, and later reconstructed back into its original form.

Python provides the json module to handle JSON serialization and deserialization. We can use the json.dumps() method for serialization in this module.

You can serialize the following Python object types into JSON strings −

  • dict
  • list
  • tuple
  • str
  • int
  • float
  • bool
  • None

Example

Following a basic example of how to serialize a Python dictionary into a JSON string −

import json

# Python dictionary
data = {"name": "Alice", "age": 30, "city": "New York"}

# Serialize to JSON string
json_string = json.dumps(data)
print(json_string)

It will produce the following output −

{"name": "Alice", "age": 30, "city": "New York"}

JSON Deserialization

JSON deserialization is the process of converting a JSON string back into a Python object. This is essential for reading and processing data that has been transmitted or stored in JSON format.

In Python, we can use json.loads() method to deserialize JSON data from a string, and json.load() method to deserialize JSON data from a file.

Example: Deserialize JSON string to Python object

In the following example we are deserializing a JSON string into a Python dictionary using the json.loads() method −

import json

# JSON string
json_string = '{"name": "John", "age": 30, "is_student": false, "courses": ["Math", "Science"], "address": {"city": "New York", "state": "NY"}}'

# Deserialize JSON string to Python object
python_obj = json.loads(json_string)

print(python_obj)

Following is the output of the above code −

{'name': 'John', 'age': 30, 'is_student': False, 'courses': ['Math', 'Science'], 'address': {'city': 'New York', 'state': 'NY'}}

Example: Deserialize JSON from File

Now, to read and deserialize JSON data from a file, we use the json.load() method −

import json

# Read and deserialize from file
with open("data.json", "r") as f:
   python_obj = json.load(f)

print(python_obj)

Output of the above code is as follows −

{'name': 'John', 'age': 30, 'is_student': False, 'courses': ['Math', 'Science'], 'address': {'city': 'New York', 'state': 'NY'}}

Advanced JSON Handling

If your JSON data includes objects that need special handling (e.g., custom classes), you can define custom deserialization functions. Use the object_hook parameter of json.loads() or json.load() method to specify a function that will be called with the result of every JSON object decoded.

Example

In the example below, we are demonstrating the usage of custom object serialization −

import json
from datetime import datetime

# Custom deserialization function
def custom_deserializer(dct):
   if 'joined' in dct:
      dct['joined'] = datetime.fromisoformat(dct['joined'])
   return dct

# JSON string with datetime
json_string = '{"name": "John", "joined": "2021-05-17T10:15:00"}'

# Deserialize with custom function
python_obj = json.loads(json_string, object_hook=custom_deserializer)

print(python_obj)

We get the output as shown below −

{'name': 'John', 'joined': datetime.datetime(2021, 5, 17, 10, 15)}

JSONEncoder Class

The JSONEncoder class in Python is used to encode Python data structures into JSON format. Each Python data type is converted into its corresponding JSON type, as shown in the following table −

Python JSON
Dict object
list, tuple array
Str string
int, float, int- & float-derived Enums number
True true
False false
None null

The JSONEncoder class is instantiated using the JSONEncoder() constructor. The following important methods are defined in this class −

  • encode(obj) − Serializes a Python object into a JSON formatted string.

  • iterencode(obj) − Encodes the object and returns an iterator that yields the encoded form of each item in the object.

  • indent − Determines the indent level of the encoded string.

  • sort_keys − If True, the keys appear in sorted order.

  • check_circular − If True, checks for circular references in container-type objects.

Example

In the following example, we are encoding Python list object. We use the iterencode() method to display each part of the encoded string −

import json

data = ['Rakesh', {'marks': (50, 60, 70)}]
e = json.JSONEncoder()

# Using iterencode() method 
for obj in e.iterencode(data):
   print(obj)

It will produce the following output −

["Rakesh"
, 
{
"marks"
: 
[50
, 60
, 70
]
}
]

JSONDecoder class

The JSONDecoder class is used to decode a JSON string back into a Python data structure. The main method in this class is decode().

Example

In this example, the "JSONEncoder" is used to encode a Python list into a JSON string, and the "JSONDecoder" is then used to decode the JSON string back into a Python list −

import json

data = ['Rakesh', {'marks': (50, 60, 70)}]
e = json.JSONEncoder()
s = e.encode(data)
d = json.JSONDecoder()
obj = d.decode(s)
print(obj, type(obj))

The result obtained is as shown below −

['Rakesh', {'marks': [50, 60, 70]}] <class 'list'>
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