TypeError: Object of type datetime is not JSON serializable

You’re trying to convert a Python object to JSON that json.dumps() doesn’t know how to handle.

Why this happens

Python’s json module only serializes basic types: str, int, float, bool, list, dict, and None. Any other type — datetime, set, Decimal, UUID — raises this error because the serializer doesn’t know how to represent them as JSON.

Fix 1: Convert to a serializable type

import json
from datetime import datetime

data = {"created": datetime.now()}

# ❌ datetime isn't serializable
json.dumps(data)

# ✅ Convert to string first
data = {"created": datetime.now().isoformat()}
json.dumps(data)

Fix 2: Custom encoder

class CustomEncoder(json.JSONEncoder):
    def default(self, obj):
        if isinstance(obj, datetime):
            return obj.isoformat()
        if isinstance(obj, set):
            return list(obj)
        return super().default(obj)

json.dumps(data, cls=CustomEncoder)

Fix 3: Use default parameter

json.dumps(data, default=str)  # Converts everything to string

Alternative solution: Use Pydantic for complex objects

Pydantic models handle datetime, UUID, etc. natively:

from pydantic import BaseModel

class Event(BaseModel):
    name: str
    created: datetime

print(Event(name="deploy", created=datetime.now()).model_dump_json())

Prevention

• Build a project-wide custom encoder early and reuse it across your codebase.
• Convert ORM query results to dicts or Pydantic models before serializing.

Related: Python cheat sheet · Python complete guide · Pip Install Error Fix · Unexpected End of JSON Input Fix