UnboundLocalError: local variable 'conn' referenced before assignment

Question

I have an error (shown in title) which occurs when I run this script:

import psycopg2

conn                =  None
conn_string         = "host='localhost' dbname='localdb' user='someuser' password='abracadabra'"


def connectDb():
    if conn is not None:   # Error occurs on this line
        return

    # print the connection string we will use to connect
    print "Connecting to database\n ->%s" % (conn_string)

conn has global scope, and is assigned to None before being referenced in the function - why the error message?

You haven't pasted in the whole function body. The problem arises because you are rebinding the variable later on in this scope — John La Rooy, Dec 21 '11 at 09:42

score 13 · Accepted Answer · answered Dec 21 '11 at 09:33

13

In python you have to declare your global variables which you want to alter in functions with the global keyword:

def connectDb():
    global conn
    if conn is not None:   # Error occurs on this line
        return
    ...

My guess is that you are going to assign some value to conn somewhere later in the function, so you have to use the global keyword.

answered Dec 21 '11 at 09:33

Constantinius

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Wow, I never seen that before - looks kinda like PHP :) – Homunculus Reticulli Dec 21 '11 at 09:34
That is only required if you wish to rebind the variable – John La Rooy Dec 21 '11 at 09:34
@gnibbler: yes of course. But I think that is what OP is doing later in the function. – Constantinius Dec 21 '11 at 09:35
1

I think you are correct. It is confusing the first time you see it because the error comes from the first time the variable is used rather than when it is assigned to – John La Rooy Dec 21 '11 at 09:41

UnboundLocalError: local variable 'conn' referenced before assignment

1 Answers1