Accessing variable from enclosing scope in nested function

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

Please explain the reason for doing it. This will help us to give better solutions — Abhijit
– Abhijit, Commented Apr 17, 2012 at 15:06

ch3ka · Accepted Answer · 2012-04-17 15:06:06Z

8

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.

answered Apr 17, 2012 at 15:06

ch3ka

12.2k5 gold badges33 silver badges28 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

moooeeeep Over a year ago

apparantly nonlocal is only available for Python 3.x , but this was a great search keyword.

jsbueno · Accepted Answer · 2012-04-17 15:07:47Z

3

Here is your work around:

class Test(object):
    def __init__(self):
        i = [0]
        def foo():
            i[0] += 1
        foo()
        print i[0]

t = Test()

This would be a use case for the nonlocal keyword, instead of global but that is only available as of Python 3.

answered Apr 17, 2012 at 15:07

jsbueno

113k11 gold badges159 silver badges239 bronze badges

3 Comments

moooeeeep Over a year ago

+1 for the Python 2.x workaround. Surprisingly container types are unaffected from this.

ch3ka Over a year ago

They are affected. What saves you here, is that the scoping rules DO allow READ access. Technically, you do not WRITE to i, you write to i[0]...

moooeeeep Over a year ago

Interesting, found this answer to explain this very well.

Collectives™ on Stack Overflow

Accessing variable from enclosing scope in nested function

2 Answers 2

1 Comment

3 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

3 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related