Python json.JSONDecoder.__init__() Method

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The Python json.JSONDecoder.__init__() method initializes a JSONDecoder instance with optional custom parsing behavior. It allows customization of how JSON data is decoded into Python objects.

This method is called internally when creating an instance of JSONDecoder.

Syntax

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

JSONDecoder(__init__(object_hook=None, object_pairs_hook=None, parse_float=None, parse_int=None, parse_constant=None, strict=True))

Parameters

This method accepts the following parameters −

• object_hook (optional): A function to customize how dictionaries are decoded.
• object_pairs_hook (optional): A function to customize how ordered key-value pairs are decoded.
• parse_float (optional): A function that processes floating-point numbers.
• parse_int (optional): A function that processes integer numbers.
• parse_constant (optional): A function that processes special constants like NaN, Infinity, and -Infinity.
• strict (optional, default=True): If False, allows non-standard JSON (e.g., trailing commas).

Return Value

This method does not return a value. It initializes a JSONDecoder instance with the specified parameters.

Example: Creating a Default JSONDecoder

In this example, we create a default instance of JSONDecoder and use it to decode a JSON string −

import json

# Create a JSONDecoder instance
decoder = json.JSONDecoder()

# JSON string
json_data = '{"name": "Alice", "age": 25}'

# Decode JSON
result = decoder.decode(json_data)

print("Decoded Data:", result)

Following is the output of the above code −

Decoded Data: {'name': 'Alice', 'age': 25}

Example: Customize Decoding

We can use the object_hook parameter to modify how JSON objects are decoded −

import json

# Custom function to modify JSON object keys
def custom_decoder(obj):
   return {key.upper(): value for key, value in obj.items()}

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

# JSON string
json_data = '{"name": "Alice", "age": 25}'

# Decode JSON with custom function
result = decoder.decode(json_data)

print("Transformed Data:", result)

Following is the output obtained −

Transformed Data: {'NAME': 'Alice', 'AGE': 25}

Example: Parsing Floating-Point and Integer Numbers

We can use parse_float and parse_int parameters to change how numbers are interpreted −

import json

# Custom parsing functions
def parse_int_custom(value):
   return int(value) * 10

def parse_float_custom(value):
   return round(float(value), 2)

# Create a JSONDecoder with custom number parsing
decoder = json.JSONDecoder(parse_int=parse_int_custom, parse_float=parse_float_custom)

# JSON string
json_data = '{"integer": "5", "floating": "12.678"}'

# Decode JSON
result = decoder.decode(json_data)

print("Parsed Numbers:", result)

We get the output as shown below −

Parsed Numbers: {'integer': 50, 'floating': 12.68}

Example: Handling Special Constants (NaN, Infinity)

Using parse_constant, we can control how special constants are processed −

import json

# Custom function for special constants
def parse_constant_custom(value):
   return f"Constant: {value}"

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

# JSON string
json_data = '{"value": NaN}'

# Decode JSON
result = decoder.decode(json_data)

print("Decoded Special Constants:", result)

The result obtained is as shown below −

Decoded Special Constants: {'value': 'Constant: NaN'}