Question

I have nested a function definition bar() inside another function foo(). Now I am trying to access a variable located in the outer function foo() from the nested function bar(). This however doesn’t work because of the scoping rules (see error traceback below).

I am looking for something similar to the global keyword, which however only enables me to access global variables, whereas this is some kind of semi-global variable.

Here’s example code:

def foo():
    i = 0
    def bar():
        # how can I get access to the variable from enclosing scope?
        i += 1
    bar()

foo()

The output is:

$ python test.py
Traceback (most recent call last):
  File "test.py", line 7, in <module>
    foo()
  File "test.py", line 5, in foo
    bar()
  File "test.py", line 4, in bar
    i += 1
UnboundLocalError: local variable 'i' referenced before assignment

Answer 1

You need the nonlocal statement instead of global.

i is clearly not global, but it is also not local to foo. It is local to __init__. Thus, in order to access it, declare it nonlocal.

Unfortunately, nonlocal ist python3 only.
You can simulate it via closure, but that’d get pretty ugly.