Show a simple example of a fastapi route that returns a simple value, or responds with an error. Be sure to use best practices with the response. I want to learn how to return a response correctly from a route.

Of course! When writing FastAPI routes, the most robust way to return responses is to:

• Use type hints for return values.
• Return Pydantic models or dicts for successful responses.
• Raise HTTPException for error responses (so FastAPI properly sets the HTTP status code and error details).

Let’s see a minimal, well-structured example that demonstrates both a successful return and an error return:

from fastapi import FastAPI, HTTPException
from pydantic import BaseModel

app = FastAPI()


class ItemResponse(BaseModel):
    message: str
    value: int


@app.get("/item/{item_id}", response_model=ItemResponse)
async def get_item(item_id: int):
    # Example: Only allow item_id == 1 for demo purposes
    if item_id != 1:
        # Responds with 404 and a JSON error
        raise HTTPException(status_code=404, detail="Item not found!")

    # Successful response using the Pydantic model
    return ItemResponse(message="Here is your item!", value=item_id)

Key Points:

• Best Practice: Use response_model in your route decorator so FastAPI validates and documents your output.
• Recommended: Return a Pydantic model instance for successful data.
• For errors: Use raise HTTPException(...) with the desired status code and detail message.
• Automatic: FastAPI handles converting your object to JSON and the correct HTTP status code.

Testing:

• /item/1 returns:

  {
    "message": "Here is your item!",
    "value": 1
  }
  

• /item/2 returns:

  {
    "detail": "Item not found!"
  }
  

  (with status code 404)

Let me know if you want more about custom error handling, custom responses, or additional FastAPI tips!

Show me a list of commonly used status codes.

Absolutely! Here’s a practical list of HTTP status codes commonly used in FastAPI (and most APIs), along with their typical purpose:

Common HTTP Status Codes

How to use in FastAPI

For manual error responses, use:

from fastapi import HTTPException

raise HTTPException(status_code=404, detail="Not found")

Or set the status code for success responses:

from fastapi import status

@app.post("/items", status_code=status.HTTP_201_CREATED)
async def create_item(...):
    ...

Tip:
FastAPI includes these status codes in fastapi.status for convenience:

from fastapi import status

status.HTTP_200_OK
status.HTTP_201_CREATED
status.HTTP_400_BAD_REQUEST
# etc.

Let me know if you want a full list or examples of custom error handling!