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Python json.JSONEncoder.default() Method
The Python json.JSONEncoder.default() method is a method of the json.JSONEncoder class that can be overridden to customize how non-serializable objects are encoded into JSON.
By default, the default() method raises a TypeError if an object is not serializable. However, we can override this method to define custom serialization logic for unsupported data types.
Syntax
Following is the syntax of the Python json.JSONEncoder.default() method −
json.JSONEncoder().default(obj)
Parameters
This method accepts the Python object as a parameter that needs to be serialized.
Return Value
This method should return a JSON-serializable object. If an object cannot be serialized, a TypeError is raised unless handled in a custom implementation.
Example: Default Behavior of default()
If we try to serialize an unsupported object, Python will raise a TypeError by default −
import json # Sample class class Person: def __init__(self, name, age): self.name = name self.age = age # Create a Person object person = Person("Alice", 30) # Attempt to encode the object try: json_data = json.dumps(person) except TypeError as e: print("Error:", e)
Following is the output obtained −
Error: Object of type 'Person' is not JSON serializable
Example: Custom Serialization Using default()
We can override the default() method to convert an unsupported object into a JSON-compatible format −
import json # Sample class class Person: def __init__(self, name, age): self.name = name self.age = age # Custom JSON Encoder class CustomEncoder(json.JSONEncoder): def default(self, obj): if isinstance(obj, Person): return {"name": obj.name, "age": obj.age} # Convert to dictionary return super().default(obj) # Fallback to default behavior # Create a Person object person = Person("Alice", 30) # Encode the object using the custom encoder json_data = json.dumps(person, cls=CustomEncoder) print("Serialized JSON:", json_data)
We get the output as shown below −
Serialized JSON: {"name": "Alice", "age": 30}
Example: Handling Multiple Custom Types
We can modify the default() method to support multiple custom types −
import json from datetime import datetime # Sample classes class Person: def __init__(self, name, age): self.name = name self.age = age class Event: def __init__(self, event_name, date): self.event_name = event_name self.date = date # datetime object # Custom JSON Encoder class CustomEncoder(json.JSONEncoder): def default(self, obj): if isinstance(obj, Person): return {"name": obj.name, "age": obj.age} elif isinstance(obj, Event): return {"event_name": obj.event_name, "date": obj.date.isoformat()} # Convert datetime to string return super().default(obj) # Create objects person = Person("Bob", 25) event = Event("Conference", datetime(2025, 5, 17)) # Encode the objects json_data = json.dumps({"person": person, "event": event}, cls=CustomEncoder) print("Serialized JSON:", json_data)
The result produced is as shown below −
Serialized JSON: {"person": {"name": "Bob", "age": 25}, "event": {"event_name": "Conference", "date": "2025-05-17T00:00:00"}}
Example: Using default() with JSONEncoder Subclass
Instead of creating a custom encoder class, we can pass a function to the default() method directly using the default parameter in json.dumps() function −
import json # Sample class class Product: def __init__(self, name, price): self.name = name self.price = price # Custom serialization function def custom_serializer(obj): if isinstance(obj, Product): return {"product_name": obj.name, "product_price": obj.price} raise TypeError(f"Object of type {obj.__class__.__name__} is not JSON serializable") # Create a Product object product = Product("Laptop", 1200.50) # Encode the object with custom serialization function json_data = json.dumps(product, default=custom_serializer) print("Serialized JSON:", json_data)
After executing the above code, we get the following output −
Serialized JSON: {"product_name": "Laptop", "product_price": 1200.5}
Example: Handling Nested Objects
We can also handle nested objects inside dictionaries or lists using the default() method −
import json # Sample class class Address: def __init__(self, street, city): self.street = street self.city = city class Person: def __init__(self, name, age, address): self.name = name self.age = age self.address = address # Nested object # Custom JSON Encoder class CustomEncoder(json.JSONEncoder): def default(self, obj): if isinstance(obj, Address): return {"street": obj.street, "city": obj.city} elif isinstance(obj, Person): return {"name": obj.name, "age": obj.age, "address": self.default(obj.address)} return super().default(obj) # Create objects address = Address("123 Main St", "New York") person = Person("Alice", 30, address) # Encode the object json_data = json.dumps(person, cls=CustomEncoder) print("Serialized JSON:", json_data)
Following is the output of the above code −
Serialized JSON: {"name": "Alice", "age": 30, "address": {"street": "123 Main St", "city": "New York"}}