How do I integrate custom exception handling with the FastAPI exception handling?
Question:
Python version 3.9, FastAPI version 0.78.0
I have a custom function that I use for application exception handling. When requests run into internal logic problems, i.e I want to send an HTTP response of 400 for some reason, I call a utility function.
@staticmethod
def raise_error(error: str, code: int) -> None:
logger.error(error)
raise HTTPException(status_code=code, detail=error)
Not a fan of this approach. So I look at
from fastapi import FastAPI, HTTPException, status
from fastapi.respones import JSONResponse
class ExceptionCustom(HTTPException):
pass
def exception_404_handler(request: Request, exc: HTTPException):
return JSONResponse(status_code=status.HTTP_404_NOT_FOUND, content={"message": "404"})
app.add_exception_handler(ExceptionCustom, exception_404_handler)
The problem I run into with the above approach is the inability to pass in the message as an argument.
Any thoughts on the whole topic?
Answers:
Your custom exception can have any custom attributes that you want. Let’s say you write it this way:
class ExceptionCustom(HTTPException):
pass
in your custom handler, you can do something like
def exception_404_handler(request: Request, exc: HTTPException):
return JSONResponse(status_code=status.HTTP_404_NOT_FOUND, content={"message": exc.detail})
Then, all you need to do is to raise the exception this way:
raise ExceptionCustom(status_code=404, detail='error message')
Note that you are creating a handler for this specific ExceptionCustom
. If all you need is the message, you can write something more generic:
class MyHTTPException(HTTPException):
pass
def my_http_exception_handler(request: Request, exc: HTTPException):
return JSONResponse(status_code=exc.status_code, content={"message": exc.detail})
app.add_exception_handler(MyHTTPException, my_http_exception_handler)
This way you can raise any exception, with any status code and any message and have the message
in your JSON response.
There’s a detailed explanation on FastAPI docs
You can add custom exception handlers, and use attributes in your Exception
class (i.e., class MyException(Exception)
in the example below) to pass any message/variables that you would like to do so. The exception handler (i.e., @app.exception_handler(MyException)
in the case below) will handle the exception as you wish and return your custom message. For more options, please have a look at this related answer as well.
Working Example
To trigger the exception in the example below, access the following URL from your browser: http://localhost:8000/something
from fastapi import FastAPI, Request, status
from fastapi.responses import JSONResponse
class MyException(Exception):
def __init__(self, name: str):
self.name = name
app = FastAPI()
@app.exception_handler(MyException)
async def my_exception_handler(request: Request, exc: MyException):
return JSONResponse(status_code=status.HTTP_404_NOT_FOUND,
content={"message": f"{exc.name} cannot be found." })
@app.get("/{name}")
def read_name(name: str):
if name == "something":
raise MyException(name=name)
return {"name": name}
In case you wouldn’t like to use the @app.exception_handler()
decorator, you could remove the decorator from the my_exception_handler()
funciton and instead use the add_exception_handler()
method to add. Example:
app.add_exception_handler(MyException, my_exception_handler)
Another way to add the exception handler to the app would be to use the exception_handlers
parameter of the FastAPI class, as demonstrated here. Related answers can also be found here and here.
Python version 3.9, FastAPI version 0.78.0
I have a custom function that I use for application exception handling. When requests run into internal logic problems, i.e I want to send an HTTP response of 400 for some reason, I call a utility function.
@staticmethod
def raise_error(error: str, code: int) -> None:
logger.error(error)
raise HTTPException(status_code=code, detail=error)
Not a fan of this approach. So I look at
from fastapi import FastAPI, HTTPException, status
from fastapi.respones import JSONResponse
class ExceptionCustom(HTTPException):
pass
def exception_404_handler(request: Request, exc: HTTPException):
return JSONResponse(status_code=status.HTTP_404_NOT_FOUND, content={"message": "404"})
app.add_exception_handler(ExceptionCustom, exception_404_handler)
The problem I run into with the above approach is the inability to pass in the message as an argument.
Any thoughts on the whole topic?
Your custom exception can have any custom attributes that you want. Let’s say you write it this way:
class ExceptionCustom(HTTPException):
pass
in your custom handler, you can do something like
def exception_404_handler(request: Request, exc: HTTPException):
return JSONResponse(status_code=status.HTTP_404_NOT_FOUND, content={"message": exc.detail})
Then, all you need to do is to raise the exception this way:
raise ExceptionCustom(status_code=404, detail='error message')
Note that you are creating a handler for this specific ExceptionCustom
. If all you need is the message, you can write something more generic:
class MyHTTPException(HTTPException):
pass
def my_http_exception_handler(request: Request, exc: HTTPException):
return JSONResponse(status_code=exc.status_code, content={"message": exc.detail})
app.add_exception_handler(MyHTTPException, my_http_exception_handler)
This way you can raise any exception, with any status code and any message and have the message
in your JSON response.
There’s a detailed explanation on FastAPI docs
You can add custom exception handlers, and use attributes in your Exception
class (i.e., class MyException(Exception)
in the example below) to pass any message/variables that you would like to do so. The exception handler (i.e., @app.exception_handler(MyException)
in the case below) will handle the exception as you wish and return your custom message. For more options, please have a look at this related answer as well.
Working Example
To trigger the exception in the example below, access the following URL from your browser: http://localhost:8000/something
from fastapi import FastAPI, Request, status
from fastapi.responses import JSONResponse
class MyException(Exception):
def __init__(self, name: str):
self.name = name
app = FastAPI()
@app.exception_handler(MyException)
async def my_exception_handler(request: Request, exc: MyException):
return JSONResponse(status_code=status.HTTP_404_NOT_FOUND,
content={"message": f"{exc.name} cannot be found." })
@app.get("/{name}")
def read_name(name: str):
if name == "something":
raise MyException(name=name)
return {"name": name}
In case you wouldn’t like to use the @app.exception_handler()
decorator, you could remove the decorator from the my_exception_handler()
funciton and instead use the add_exception_handler()
method to add. Example:
app.add_exception_handler(MyException, my_exception_handler)
Another way to add the exception handler to the app would be to use the exception_handlers
parameter of the FastAPI class, as demonstrated here. Related answers can also be found here and here.