how to get access to error message from abort command when using custom error handler

Question

Using a python flask server, I want to be able to throw an http error response with the abort command and use a custom response string and a custom message in the body

@app.errorhandler(400)
def custom400(error):
    response = jsonify({'message': error.message})
    response.status_code = 404
    response.status = 'error.Bad Request'
    return response

abort(400,'{"message":"custom error message to appear in body"}')

But the error.message variable comes up as an empty string. I can't seem to find documentation on how to get access to the second variable of the abort function with a custom error handler

Solution 1

If you look at flask/__init__.py you will see that abort is actually imported from werkzeug.exceptions. Looking at the Aborter class, we can see that when called with a numeric code, the particular HTTPException subclass is looked up and called with all of the arguments provided to the Aborter instance. Looking at HTTPException, paying particular attention to lines 85-89 we can see that the second argument passed to HTTPException.__init__ is stored in the description property, as @dirn pointed out.

You can either access the message from the description property:

@app.errorhandler(400)
def custom400(error):
    response = jsonify({'message': error.description['message']})
    # etc.

abort(400, {'message': 'custom error message to appear in body'})

or just pass the description in by itself:

@app.errorhandler(400)
def custom400(error):
    response = jsonify({'message': error.description})
    # etc.

abort(400, 'custom error message to appear in body')

Solution 2

People rely on abort() too much. The truth is that there are much better ways to handle errors.

For example, you can write this helper function:

def bad_request(message):
    response = jsonify({'message': message})
    response.status_code = 400
    return response

Then from your view function you can return an error with:

@app.route('/')
def index():
    if error_condition:
        return bad_request('message that appears in body')

If the error occurs deeper in your call stack in a place where returning a response isn't possible then you can use a custom exception. For example:

class BadRequestError(ValueError):
    pass

@app.errorhandler(BadRequestError)
def bad_request_handler(error):
    return bad_request(str(error))

Then in the function that needs to issue the error you just raise the exception:

def some_function():
    if error_condition:
        raise BadRequestError('message that appears in the body')

I hope this helps.

Solution 3

I simply do it like this:

    abort(400, description="Required parameter is missing")

Solution 4

flask.abort also accepts flask.Response

abort(make_response(jsonify(message="Error message"), 400))

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