7. Error Handling & Exception Design

FastAPI Exception Hierarchy

FastAPI provides a structured exception hierarchy that maps Python exceptions to HTTP responses. Built-in exceptions such as HTTPException are designed to express client-facing errors with clear status codes and messages. This hierarchy encourages explicit error signaling rather than implicit failure.

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Custom Exceptions

Custom exceptions allow applications to model domain-specific error conditions explicitly. By defining meaningful exception types, error scenarios become self-documenting and easier to reason about. This approach separates business rules from HTTP mechanics while preserving clarity.

from fastapi import HTTPException

def user_not_found(user_id: int):
    raise HTTPException(status_code=404, detail="User not found")

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Centralized Error Handling

FastAPI supports centralized error handling through global exception handlers. This allows applications to standardize error responses, logging, and monitoring in a single location. Centralization improves consistency and reduces duplication across route handlers.

from fastapi import FastAPI, Request
from fastapi.responses import JSONResponse

app = FastAPI()

@app.exception_handler(ValueError)
async def value_error_handler(request: Request, exc: ValueError):
    return JSONResponse(status_code=400, content={"detail": str(exc)})

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Designing Predictable API Failures

Predictable APIs fail in consistent and documented ways. FastAPI encourages defining clear error contracts so clients can rely on structured responses rather than ad-hoc messages. This predictability is essential for building resilient client integrations and automated systems.