python type_error ai_generated true

TypeError: Object of type 'User' is not JSON serializable

ID: python/fastapi-typeerror-object-of-type-type-is-not-json-serializable

Also available as: JSON · Markdown · 中文
80%Fix Rate
87%Confidence
0Evidence
2024-03-30First Seen

Version Compatibility

VersionStatusIntroducedDeprecatedNotes
3.x active — — —

Root Cause

Returning a custom Python object directly from an endpoint without converting to a JSON-serializable format.

generic

中文

直接从端点返回自定义 Python 对象，而未转换为 JSON 可序列化格式。

Workarounds

  1. 95% success Use a Pydantic model for the response. 
    from pydantic import BaseModel
class UserOut(BaseModel):
    id: int
    name: str

@app.get('/users/{user_id}', response_model=UserOut)
async def get_user(user_id: int):
    user = get_user_from_db(user_id)
    return UserOut(id=user.id, name=user.name)
  2. 90% success Convert the object to a dictionary manually. 
    @app.get('/users/{user_id}')
async def get_user(user_id: int):
    user = get_user_from_db(user_id)
    return {'id': user.id, 'name': user.name}

Dead Ends

Common approaches that don't work:

  1. Using json.dumps() on the object directly. 70% fail

    json.dumps() also requires a serializable object; custom objects need a custom encoder.

  2. Returning the object without a response model. 80% fail

    FastAPI tries to serialize the object automatically but fails if it's not a dict or Pydantic model.