Handling Errors

There are many situations in where you need to notify an error to the client that is using your API.

This client could be a browser with a frontend, the code from someone else, an IoT device, etc.

You could need to tell that client that:

  • He doesn't have enough privileges for that operation.
  • He doesn't have access to that resource.
  • The item he was trying to access doesn't exist.
  • etc.

In these cases, you would normally return an HTTP status code in the range of 400 (from 400 to 499).

This is similar to the 200 HTTP status codes (from 200 to 299). Those "200" status codes mean that somehow there was a "success" in the request.

The status codes in the 400 range mean that there was an error from the client.

Remember all those "404 Not Found" errors (and jokes)?

Use HTTPException

To return HTTP responses with errors to the client you use HTTPException.

Import HTTPException

from fastapi import FastAPI, HTTPException

app = FastAPI()

items = {"foo": "The Foo Wrestlers"}


@app.get("/items/{item_id}")
async def read_item(item_id: str):
    if item_id not in items:
        raise HTTPException(status_code=404, detail="Item not found")
    return {"item": items[item_id]}

Raise an HTTPException in your code

HTTPException is a normal Python exception with additional data relevant for APIs.

Because it's a Python exception, you don't return it, you raise it.

This also means that if you are inside a utility function that you are calling inside of your path operation function, and you raise the HTTPException from inside of that utility function, it won't run the rest of the code in the path operation function, it will terminate that request right away and send the HTTP error from the HTTPException to the client.

The benefit of raising an exception over returning a value will be more evident in the section about Dependencies and Security.

In this example, when the client request an item by an ID that doesn't exist, raise an exception with a status code of 404:

from fastapi import FastAPI, HTTPException

app = FastAPI()

items = {"foo": "The Foo Wrestlers"}


@app.get("/items/{item_id}")
async def read_item(item_id: str):
    if item_id not in items:
        raise HTTPException(status_code=404, detail="Item not found")
    return {"item": items[item_id]}

The resulting response

If the client requests http://example.com/items/foo (an item_id "foo"), he will receive an HTTP status code of 200, and a JSON response of:

{
  "item": "The Foo Wrestlers"
}

But if the client requests http://example.com/items/bar (a non-existent item_id "bar"), he will receive an HTTP status code of 404 (the "not found" error), and a JSON response of:

{
  "detail": "Item not found"
}

Tip

When raising an HTTPException, you can pass any value that can be converted to JSON as the parameter detail, not only str.

You could pass a dict, a list, etc.

They are handled automatically by FastAPI and converted to JSON.

Add custom headers

There are some situations in where it's useful to be able to add custom headers to the HTTP error. For example, for some types of security.

You probably won't need to use it directly in your code.

But in case you needed it for an advanced scenario, you can add custom headers:

from fastapi import FastAPI, HTTPException

app = FastAPI()

items = {"foo": "The Foo Wrestlers"}


@app.get("/items-header/{item_id}")
async def read_item_header(item_id: str):
    if item_id not in items:
        raise HTTPException(
            status_code=404,
            detail="Item not found",
            headers={"X-Error": "There goes my error"},
        )
    return {"item": items[item_id]}

Install custom exception handlers

You can add custom exception handlers with the same exception utilities from Starlette.

Let's say you have a custom exception UnicornException that you (or a library you use) might raise.

And you want to handle this exception globally with FastAPI.

You could add a custom exception handler with @app.exception_handler():

from fastapi import FastAPI
from starlette.requests import Request
from starlette.responses import JSONResponse


class UnicornException(Exception):
    def __init__(self, name: str):
        self.name = name


app = FastAPI()


@app.exception_handler(UnicornException)
async def unicorn_exception_handler(request: Request, exc: UnicornException):
    return JSONResponse(
        status_code=418,
        content={"message": f"Oops! {exc.name} did something. There goes a rainbow..."},
    )


@app.get("/unicorns/{name}")
async def read_unicorn(name: str):
    if name == "yolo":
        raise UnicornException(name=name)
    return {"unicorn_name": name}

Here, if you request /unicorns/yolo, the path operation will raise a UnicornException.

But it will be handled by the unicorn_exception_handler.

So, you will receive a clean error, with an HTTP status code of 418 and a JSON content of:

{"message": "Oops! yolo did something. There goes a rainbow..."}

Override the default exception handlers

FastAPI has some default exception handlers.

These handlers are in charge or returning the default JSON responses when you raise an HTTPException and when the request has invalid data.

You can override these exception handlers with your own.

Override request validation exceptions

When a request contains invalid data, FastAPI internally raises a RequestValidationError.

And it also includes a default exception handler for it.

To override it, import the RequestValidationError and use it with @app.exception_handler(RequestValidationError) to decorate the exception handler.

The exception handler will receive a Request and the exception.

from fastapi import FastAPI, HTTPException
from fastapi.exceptions import RequestValidationError
from starlette.exceptions import HTTPException as StarletteHTTPException
from starlette.responses import PlainTextResponse

app = FastAPI()


@app.exception_handler(StarletteHTTPException)
async def http_exception_handler(request, exc):
    return PlainTextResponse(str(exc.detail), status_code=exc.status_code)


@app.exception_handler(RequestValidationError)
async def validation_exception_handler(request, exc):
    return PlainTextResponse(str(exc), status_code=400)


@app.get("/items/{item_id}")
async def read_item(item_id: int):
    if item_id == 3:
        raise HTTPException(status_code=418, detail="Nope! I don't like 3.")
    return {"item_id": item_id}

Now, if you go to /items/foo, instead of getting the default JSON error with:

{
    "detail": [
        {
            "loc": [
                "path",
                "item_id"
            ],
            "msg": "value is not a valid integer",
            "type": "type_error.integer"
        }
    ]
}

you will get a text version, with:

1 validation error
path -> item_id
  value is not a valid integer (type=type_error.integer)

RequestValidationError vs ValidationError

Warning

These are technical details that you might skip if it's not important for you now.

RequestValidationError is a sub-class of Pydantic's ValidationError.

FastAPI uses it so that, if you use a Pydantic model in response_model, and your data has an error, you will see the error in your log.

But the client/user will not see it. Instead, the client will receive an "Internal Server Error" with a HTTP status code 500.

It should be this way because if you have a Pydantic ValidationError in your response or anywhere in your code (not in the client's request), it's actually a bug in your code.

And while you fix it, your clients/users shouldn't have access to internal information about the error, as that could expose a security vulnerability.

Override the HTTPException error handler

The same way, you can override the HTTPException handler.

For example, you could want to return a plain text response instead of JSON for these errors:

from fastapi import FastAPI, HTTPException
from fastapi.exceptions import RequestValidationError
from starlette.exceptions import HTTPException as StarletteHTTPException
from starlette.responses import PlainTextResponse

app = FastAPI()


@app.exception_handler(StarletteHTTPException)
async def http_exception_handler(request, exc):
    return PlainTextResponse(str(exc.detail), status_code=exc.status_code)


@app.exception_handler(RequestValidationError)
async def validation_exception_handler(request, exc):
    return PlainTextResponse(str(exc), status_code=400)


@app.get("/items/{item_id}")
async def read_item(item_id: int):
    if item_id == 3:
        raise HTTPException(status_code=418, detail="Nope! I don't like 3.")
    return {"item_id": item_id}

FastAPI's HTTPException vs Starlette's HTTPException

FastAPI has its own HTTPException.

And FastAPI's HTTPException error class inherits from Starlette's HTTPException error class.

The only difference, is that FastAPI's HTTPException allows you to add headers to be included in the response.

This is needed/used internally for OAuth 2.0 and some security utilities.

So, you can keep raising FastAPI's HTTPException as normally in your code.

But when you register an exception handler, you should register it for Starlette's HTTPException.

This way, if any part of Starlette's internal code, or a Starlette extension or plug-in, raises an HTTPException, your handler will be able to catch handle it.

In this example, to be able to have both HTTPExceptions in the same code, Starlette's exceptions is renamed to StarletteHTTPException:

from starlette.exceptions import HTTPException as StarletteHTTPException

Re-use FastAPI's exception handlers

You could also just want to use the exception somehow, but then use the same default exception handlers from FastAPI.

You can import and re-use the default exception handlers from fastapi.exception_handlers:

from fastapi import FastAPI, HTTPException
from fastapi.exception_handlers import (
    http_exception_handler,
    request_validation_exception_handler,
)
from fastapi.exceptions import RequestValidationError
from starlette.exceptions import HTTPException as StarletteHTTPException

app = FastAPI()


@app.exception_handler(StarletteHTTPException)
async def custom_http_exception_handler(request, exc):
    print(f"OMG! An HTTP error!: {exc}")
    return await http_exception_handler(request, exc)


@app.exception_handler(RequestValidationError)
async def validation_exception_handler(request, exc):
    print(f"OMG! The client sent invalid data!: {exc}")
    return await request_validation_exception_handler(request, exc)


@app.get("/items/{item_id}")
async def read_item(item_id: int):
    if item_id == 3:
        raise HTTPException(status_code=418, detail="Nope! I don't like 3.")
    return {"item_id": item_id}

In this example, you are just printing the error with a very expressive notification.

But you get the idea, you can use the exception and then just re-use the default exception handlers.