how handle a variable if it's returning none

Question

Hi I have the following function to get sql from my template. variable row is fetching the query which is input by the user. If the user input an invalid sql am getting an error as UnboundLocalError: local variable 'row' referenced before assignment (Because row is none bacuse the sql is wrong) How can I handle this error effectively? Am bit new to django python. Can help me on this guys? Thanks in advance.

def DBQuery(sql):
        c = MySQLdb.connect(host=HOST,user=USER,passwd=PASS,db=DB, cursorclass=MySQLdb.cursors.DictCursor)
        cursor  = c.cursor()

        try:
            cursor.execute(sql)
            row = cursor.fetchall()
        except Exception, e:
            print "Error found!", e

        cursor.close()
        c.close()
        return row

Well, what do you *want* to happen when the SQL is invalid? – ysth May 25 '16 at 08:27 — ysth, May 25 '16 at 08:27

score 0 · Accepted Answer · answered May 25 '16 at 08:35

Declarete variable before return, something like:

def DBQuery(sql):
    c = MySQLdb.connect(host=HOST,user=USER,passwd=PASS,db=DB, cursorclass=MySQLdb.cursors.DictCursor)
    cursor  = c.cursor()
    row = None
    try:
        cursor.execute(sql)
        row = cursor.fetchall()
    except Exception, e:
        print "Error found!", e

    cursor.close()
    c.close()
    return row

score 0 · Answer 2 · answered May 25 '16 at 08:35

def DBQuery(sql):
        c = MySQLdb.connect(host=HOST,user=USER,passwd=PASS,db=DB, cursorclass=MySQLdb.cursors.DictCursor)
        cursor  = c.cursor()

        try:
            cursor.execute(sql)
            row = cursor.fetchall()
        except Exception, e:
            print "Error found!", e
            row="None"

        cursor.close()
        c.close()
        return row
#then if the Exception ocure, the func will ret the string "None"

Returning a string instead of a rowset is a very bad advice... — Selcuk, May 25 '16 at 08:36

score 0 · Answer 3 · answered May 25 '16 at 08:37

I would change the code slightly to see if cursor.execute() has returned anything before doing a cursor.fetchall().

rows_affected = cursor.execute(sql)
if rows_affected == 0:
    flag_an_error()
else:
    rows = ......

You can handle the error as appropriate to your application.

how handle a variable if it's returning none

3 Answers3