Returning API Error Messages with Python and Flask

Question

I am designing a RESTful API using Python and Flask. As expected, the API needs to receive an API request and return data if all goes well, but in the instance of an error, it needs to fail softly and return the proper error. I typically raise exceptions when an error results, but in this case I need to return the error message to the user (try-catch block?).

The way I am currently dealing with errors is to have my functions return both the data and an error, and to check for the data at each level, finally returning either the data or the error to the caller of the API function.

The problem with this is that it can get cumbersome when there are several levels of function calls, requiring my functions to pass data and errors several times and perform checks each time.

Is there a better way to do this? What are some improvements I could make to make the error propagation more simple and elegant?

Here is an example of the way I'm currently returning errors:

def get_data()
    data1, error = get_some_data() # function not shown
    if data1 is None:
         return None, "could not retrieve data1"
    data2, error = get_some_other_data() # function not shown
    if data2 is None:
         return None, "could not retrieve data2"
    return (data1, data2), None

@app.route("/api/method", methods=['GET'])
def method():
    data, error = get_data()
    if data is None:
        if error is None:
            error = "unknown error"
        return json.dumps({ "error": error }), 500
    return json.dumps({ "data": data }), 200

Solution 1

You could use abort(http_code) to return an appropriate http code to the client or just raise a non-http exception. And use @app.errorhandler() decorator to provide a custom handler for http errors and arbitrary exceptions. You could also use an ordinary try/except block where you are ready to handle an exception. Python is not Go you can use exceptions.

Solution 2

Maybe the Flask API is more suitable for your needs, as it is particularly designed for RESTful APIs.

It has better exception handling than Flask, see: http://www.flaskapi.org/api-guide/exceptions/