Python json.JSONDecoder.decode() Method

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The Python json.JSONDecoder.decode() method is used to decode a JSON-encoded string into a corresponding Python object.

This method is useful when manually parsing JSON data, especially when working with custom deserialization logic.

Syntax

Following is the syntax of the Python json.JSONDecoder.decode() method −

json.JSONDecoder().decode(s)

Parameters

This method accepts a JSON-encoded string as a parameter that needs to be decoded.

Return Value

This method returns a Python object that represents the parsed JSON data.

Example: Basic Usage of decode()

In this example, we use the json.JSONDecoder.decode() method to manually decode a JSON string −

import json

# JSON string
json_string = '{"name": "Alice", "age": 25, "city": "New York"}'

# Create JSONDecoder instance
decoder = json.JSONDecoder()

# Decode JSON string
data = decoder.decode(json_string)

print("Decoded Data:", data)

Following is the output obtained −

Decoded Data: {'name': 'Alice', 'age': 25, 'city': 'New York'}

Example: Using decode() with JSON Arrays

The decode() method can also handle JSON arrays and convert them into Python lists −

import json

# JSON string (array)
json_string = '[{"name": "Alice", "age": 25}, {"name": "Bob", "age": 30}]'

# Create JSONDecoder instance
decoder = json.JSONDecoder()

# Decode JSON string
data = decoder.decode(json_string)

print("Decoded List:", data)

We get the output as shown below −

Decoded List: [{'name': 'Alice', 'age': 25}, {'name': 'Bob', 'age': 30}]

Example: Handling Special Constants

The decode() method can also handle special JSON constants such as NaN and Infinity when using the parse_constant parameter −

import json

# Custom function to handle special constants
def parse_constant_custom(value):
   return f"Special value: {value}"

# JSON string with special constants
json_string = '{"value1": NaN, "value2": Infinity, "value3": -Infinity}'

# Create JSONDecoder instance with parse_constant
decoder = json.JSONDecoder(parse_constant=parse_constant_custom)

# Decode JSON string
data = decoder.decode(json_string)

print("Parsed Data:", data)

After executing the above code, we get the following output −

Parsed Data: {'value1': 'Special value: NaN', 'value2': 'Special value: Infinity', 'value3': 'Special value: -Infinity'}

Example: Custom Object Conversion

The object_hook parameter allows us to convert JSON objects into custom Python objects while decoding −

import json

# Sample class
class Person:
   def __init__(self, name, age):
      self.name = name
      self.age = age

# Custom function to decode JSON into Person objects
def custom_decoder(obj):
   return Person(obj["name"], obj["age"])

# JSON string
json_string = '{"name": "Bob", "age": 30}'

# Create JSONDecoder instance with object_hook
decoder = json.JSONDecoder(object_hook=custom_decoder)

# Decode JSON string
person = decoder.decode(json_string)

print("Decoded Person:", vars(person))

Following is the output of the above code −

Decoded Person: {'name': 'Bob', 'age': 30}

Example: Preserving Key Order

By default, Python dictionaries maintain order in Python 3.7+, but we can explicitly use the object_pairs_hook parameter to load a JSON object while preserving key order −

import json
from collections import OrderedDict

# JSON string
json_string = '{"c": 3, "b": 2, "a": 1}'

# Create JSONDecoder instance with object_pairs_hook
decoder = json.JSONDecoder(object_pairs_hook=OrderedDict)

# Decode JSON string
data = decoder.decode(json_string)

print("Ordered Data:", data)

The result produced is as follows −

Ordered Data: OrderedDict({'c': 3, 'b': 2, 'a': 1})