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

Because that will return an internal server error without a proper message in json format. Think Facebook API style. — Ryan Shea
– Ryan Shea, Commented Sep 27, 2012 at 21:43
I kind of agree with @thg435. Seems to me that a try...except would clean up your "method" a bit. You would just move your error handling block to the "except" block and can still return the error message. This is how I do it in Django with Ajax calls. Also, I don't have any experience with Flask, but if this is a RESTful(ish) API. and it appears to be because you are going a "GET", you should return an appropriate status code. 200 if things are ok. Or a 4XX or 500 (depending on what the underlying problem is). — David S
– David S, Commented Sep 27, 2012 at 21:45
My bad on the status code, I forgot to include that in my example. I just fixed it. The default is to return a status code of 200. — Ryan Shea
– Ryan Shea, Commented Sep 27, 2012 at 21:46
Ah ok, I believe I misunderstood at first. I could try that as well. Not sure whether that is more or less elegant than J.F.'s solution. — Ryan Shea
– Ryan Shea, Commented Sep 27, 2012 at 21:57

jfs · Accepted Answer · 2012-09-27 21:43:55Z

47

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.

answered Sep 27, 2012 at 21:43

jfs

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3 Comments

Ryan Shea Over a year ago

I like the raise non-http exception plus @app.errorhandler() idea. I'm going to look into it in the Flask documentation and then see how well it works.

D.Shawley Over a year ago

+1 - errorhandler and throwing exceptions really is the correct answer

Pitt Over a year ago

Flask docs explicitly recommend avoiding the use of abort in favour of writing your own exception class and error handler -> flask.pocoo.org/docs/0.12/patterns/apierrors

Mathieu Rodic · Accepted Answer · 2014-11-21 07:02:12Z

4

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/

answered Nov 21, 2014 at 7:02

Mathieu Rodic

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