python 34 lines · 7 steps

Custom exception handlers in FastAPI

Map a domain-specific exception to a structured HTTP error response so route handlers can fail by raising, not by formatting JSON.

Explained by highlit
FastAPI Intermediate exception handling error responses separation of concerns domain modeling
1from fastapi import FastAPI, Request, status
2from fastapi.responses import JSONResponse
3 
4app = FastAPI()
5 
6 
7class InsufficientFundsError(Exception):
8 def __init__(self, account_id: str, balance: float, requested: float):
9 self.account_id = account_id
10 self.balance = balance
11 self.requested = requested
12 
13 
14@app.exception_handler(InsufficientFundsError)
15async def insufficient_funds_handler(request: Request, exc: InsufficientFundsError):
16 return JSONResponse(
17 status_code=status.HTTP_402_PAYMENT_REQUIRED,
18 content={
19 "error": "insufficient_funds",
20 "message": f"Account {exc.account_id} cannot withdraw {exc.requested:.2f}.",
21 "available_balance": exc.balance,
22 "shortfall": round(exc.requested - exc.balance, 2),
23 },
24 headers={"X-Error-Code": "INSUFFICIENT_FUNDS"},
25 )
26 
27 
28@app.post("/accounts/{account_id}/withdraw")
29async def withdraw(account_id: str, amount: float):
30 account = await accounts.get(account_id)
31 if amount > account.balance:
32 raise InsufficientFundsError(account_id, account.balance, amount)
33 await account.debit(amount)
34 return {"account_id": account_id, "balance": account.balance - amount}
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Centralizing error formatting in a handler keeps route logic focused on the happy path.
  2. 2A custom exception can carry rich context that the handler turns into a meaningful response.
  3. 3Mapping domain failures to specific status codes makes an API's errors predictable for clients.

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