Python json._default_decoder Attribute

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The Python json._default_decoder attribute refers to the default instance of json.JSONDecoder, which is used internally by the json module for decoding JSON strings into Python objects.

It provides a standard way to decode JSON strings without needing to manually instantiate a JSONDecoder object.

Syntax

Following is the syntax of using json._default_decoder −

import json

decoded_data = json._default_decoder.decode(json_string)

Return Value

The json._default_decoder returns a Python object parsed from the given JSON string.

Example: Decoding JSON

The following example demonstrates how to decode JSON using the json._default_decoder attribute −

import json

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

# Decode JSON using _default_decoder
decoded_data = json._default_decoder.decode(json_string)

print("Decoded JSON:", decoded_data)

Following is the output obtained −

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

Example: Handling Invalid JSON

If the JSON string is malformed, the json._default_decoder attribute raises a JSONDecodeError −

import json

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

try:
   # Attempt to decode invalid JSON
   decoded_data = json._default_decoder.decode(invalid_json)
   print("Decoded Data:", decoded_data)
except json.JSONDecodeError as e:
   print("Error:", e)

Following is the output of the above code −

Error: Expecting property name enclosed in double quotes: line 1 column 29 (char 28)

Example: Decoding JSON Arrays

The json._default_decoder attribute can also decode JSON arrays into Python lists −

import json

# JSON array string
json_array_string = '[10, 20, 30, 40]'

# Decode JSON array
decoded_array = json._default_decoder.decode(json_array_string)

print("Decoded Array:", decoded_array)

We get the output as shown below −

Decoded Array: [10, 20, 30, 40]

Example: Using _default_decoder with Custom Decoding

By subclassing JSONDecoder, we can customize the behavior of the json._default_decoder attribute −

import json

# Custom JSONDecoder to capitalize string values
class CustomDecoder(json.JSONDecoder):
   def decode(self, s, **kwargs):
      data = super().decode(s, **kwargs)
      if isinstance(data, dict):
         return {k: v.upper() if isinstance(v, str) else v for k, v in data.items()}
      return data

# Override default decoder
json._default_decoder = CustomDecoder()

# JSON string
json_string = '{"greeting": "hello", "name": "bob"}'

# Decode JSON using custom _default_decoder
decoded_data = json._default_decoder.decode(json_string)

print("Custom Decoded JSON:", decoded_data)

The result produced is as shown below −

Custom Decoded JSON: {'greeting': 'HELLO', 'name': 'BOB'}