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

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')

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.

I simply do it like this:

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