Python - Flask Api exception

Question

i am currently working on my api and wanted to implement a mechanism that i can raise a custom exception at any point and return a json response:

class InvalidUsage(Exception):
    def __init__(self, message):
        Exception.__init__(self)
        self.message = message

    def to_json(self):
        return JsonResponse(False, self.message)()

class JsonResponse(object):
    def __init__(self, success, data):
        self.success = success
        self.data = data

        if not success:
            self.result = {
                'success': self.success,
                'error': self.data
            }
        else:
            self.result = {
                'success': self.success,
                'data': self.data
            }

    def __call__(self):
        return jsonify({'result': self.result})

When i try to use it in any controller:

raise InvalidUsage('Some error ocurred').to_json()

it just prints a Traceback back to:

raise InvalidUsage('Some error ocurred').to_json()

But JsonResponse works perfectly fine. When i run:

return JsonResponse(False, 'Some error ocurred')()

it returns correct json to my browser.

But i need to get my exception raise somehow working.. so i can raise api exceptions at any point in my python-webapp(controller, service, validator, etc..)

Anybody could help me to figure out how to do this?

Thanks and Greetings!

Flask specifically states that a custom error without an exception handler would cause a 500 server error: flask.pocoo.org/docs/0.12/patterns/apierrors Have you tried implementing the handler and raising just the error, without calling .to_json() on it? I'm also not sure I understand why you would call .to_json() when raising the exception. — Alex Bausk
– Alex Bausk, Commented Jul 1, 2017 at 13:48
i implemented the exact same like in the documentation, but with same result — Creative crypter
– Creative crypter, Commented Jul 1, 2017 at 13:51
Why do you call .to_json() then? I mean probably what you should do is call to_json from the error handler, not from the raise instruction — Alex Bausk
– Alex Bausk, Commented Jul 1, 2017 at 13:53
I don't see how your raise instruction could possibly work since raise accepts an object inheriting from BaseException and your are giving it a flask Response via jsonify — Alex Bausk
– Alex Bausk, Commented Jul 1, 2017 at 14:04

darksky · Accepted Answer · 2017-07-02 02:35:28Z

Firstly, if you want to call the constructor of a parent class, you should call super

class InvalidUsage(Exception):
    def __init__(self, message):
        super(InvalidUsage, self).__init__()
        self.message = message

Second, overriding __call__ is just ugly and a bad idea. You should either implement an explicit method, or define a custom __repr__ since JSON is just a string.

Third,

self.result = {
    'success': self.success,
    'data': self.data
}

is executed on both if and else. Why do you even have an if? Why not:

class JsonResponse(object):
    def __init__(self, success, data):
        self.success = success
        self.data = data

    def __repr__(self):
        return json.dumps({'success': self.success, 'data': self.data})

which makes this class more pointless than ever if it isn't used anywhere else in the code.

Fourth, when you want to raise a custom exception, you should raise the class InvalidUsage not a particular method in it.

raise InvalidUsage("terrible usage")

You should really think clearly about what you're doing here. You're throwing Exceptions. You can't throw an instance of a Flask.Response() object as an exception. What you can do is throw your custom InvalidUsage exception, catch it, and then return your json response:

class InvalidUsage(Exception)
    pass


def horrible_code():
    raise InvalidUsage("something went wrong.")


try:
    horrible_code()

except InvalidUsage as error:
    return jsonify({"success": "nope", "error_message": str(error)})

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