Error handling in Python-MySQL

Question

I am running a little webservice based on python flask, where I want to execute a small MySQL Query. When I get a valid input for my SQL query, everything is working as expected and I get the right value back. However, if the value is not stored in the database I receive a TypeError

    Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1836, in __call__
    return self.wsgi_app(environ, start_response)
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1820, in wsgi_app
    response = self.make_response(self.handle_exception(e))
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1403, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1817, in wsgi_app
    response = self.full_dispatch_request()
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1478, in full_dispatch_request
    response = self.make_response(rv)
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1566, in make_response
    raise ValueError('View function did not return a response')
ValueError: View function did not return a response

I tried to tap into error handling myself and use this code for my project, but it seems like this doesn't work properly.

#!/usr/bin/python

from flask import Flask, request
import MySQLdb

import json

app = Flask(__name__)


@app.route("/get_user", methods=["POST"])
def get_user():
    data = json.loads(request.data)
    email = data["email"]

    sql = "SELECT userid FROM oc_preferences WHERE configkey='email' AND configvalue LIKE '" + email + "%';";

    conn = MySQLdb.connect( host="localhost",
                            user="root",
                            passwd="ubuntu",
                            db="owncloud",
                            port=3306)
    curs = conn.cursor()

    try:
        curs.execute(sql)
        user = curs.fetchone()[0]
        return user
    except MySQLdb.Error, e:
        try:
            print "MySQL Error [%d]: %s" % (e.args[0], e.args[1])
            return None
        except IndexError:
            print "MySQL Error: %s" % str(e)
            return None
    except TypeError, e:
        print(e)
        return None
    except ValueError, e:
        print(e)
        return None
    finally:
        curs.close()
        conn.close()

if __name__ == "__main__":
    app.run(host="0.0.0.0", port=5000, debug=True)

Basically I just want to return a value, when everything is working properly and I want to return nothing if it isn't preferably with an error message on my server. How can I use error handling in a proper way?

EDIT Updated current code + error message.

@kuket15 tried that too before. Received a ValueError after that :D And when I used except ValueError, e: after that, I still received a ValueError. I edited my question now, so you can see my current status — BoJack Horseman
– BoJack Horseman, Commented Jun 23, 2015 at 8:32
If it is flask catching the exception, ValueError('View function did not return...') may be being raised by any existing None return.. try a dummy-string in return or an except ValueError: pass instead for comparison? Or read the exception trap code in flask, would be my suggestion; to find out wobt cause flask to raise. May have your hands tied here. — rhoyerboat
– rhoyerboat, Commented Jun 23, 2015 at 8:51
@rhoyerboat thank you for your response :D That solved my problem :) I am probably just unable to use None in this case. — BoJack Horseman
– BoJack Horseman, Commented Jun 23, 2015 at 8:58

bruno desthuilliers · Accepted Answer · 2019-10-10 07:43:53Z

First point: you have too much code in your try/except block. Better to use distinct try/except blocks when you have two statements (or two groups of statements) that may raise different errors:

try:
    try:
        curs.execute(sql)
        # NB : you won't get an IntegrityError when reading
    except (MySQLdb.Error, MySQLdb.Warning) as e:
        print(e)
        return None

    try: 
        user = curs.fetchone()[0]
        return user
    except TypeError as e:
        print(e)
        return None

finally:
    conn.close()

Now do you really have to catch a TypeError here ? If you read at the traceback, you'll notice that your error comes from calling __getitem__() on None (nb : __getitem__() is implementation for the subscript operator []), which means that if you have no matching rows cursor.fetchone() returns None, so you can just test the return of currsor.fetchone():

try:
    try:
        curs.execute(sql)
        # NB : you won't get an IntegrityError when reading
    except (MySQLdb.Error, MySQLdb.Warning) as e:
        print(e)
        return None

    row = curs.fetchone()
    if row:
        return row[0]
    return None

finally:
    conn.close()

Now do you really need to catch MySQL errors here ? Your query is supposed to be well tested and it's only a read operation so it should not crash - so if you have something going wrong here then you obviously have a bigger problem, and you don't want to hide it under the carpet. IOW: either log the exceptions (using the standard logging package and logger.exception()) and re-raise them or more simply let them propagate (and eventually have an higher level component take care of logging unhandled exceptions):

try:
    curs.execute(sql)
    row = curs.fetchone()
    if row:
        return row[0]
    return None

finally:
    conn.close()

And finally: the way you build your sql query is utterly unsafe. Use sql placeholders instead:

q = "%s%%" % data["email"].strip() 
sql = "select userid from oc_preferences where configkey='email' and configvalue like %s"
cursor.execute(sql, [q,])

Oh and yes: wrt/ the "View function did not return a response" ValueError, it's because, well, your view returns None in many places. A flask view is supposed to return something that can be used as a HTTP response, and None is not a valid option here.

The world would be a better place if our teachers explain to us like you did !

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