UnboundLocalError with nested function scopes

Question

I have code like this (simplified):

def outer():     ctr = 0      def inner():         ctr += 1      inner() 

But ctr causes an error:

Traceback (most recent call last):   File "foo.py", line 9, in <module>     outer()   File "foo.py", line 7, in outer     inner()   File "foo.py", line 5, in inner     ctr += 1 UnboundLocalError: local variable 'ctr' referenced before assignment 

How can I fix this? I thought nested scopes would have allowed me to do this. I've tried with 'global', but it still doesn't work.

Answer 1

If you're using Python 3, you can use the nonlocal statement to enable rebinding of a nonlocal name:

def outer():     ctr = 0      def inner():         nonlocal ctr         ctr += 1      inner() 

If you're using Python 2, which doesn't have nonlocal, you need to perform your incrementing without barename rebinding (by keeping the counter as an item or attribute of some barename, not as a barename itself). For example:

... ctr = [0]  def inner():     ctr[0] += 1 ... 

and of course use ctr[0] wherever you're using bare ctr now elsewhere.