Accessing Variables Defined Inside a Closure

Contributed by: David Beazley and Brian K. Jones

Difficulty level: Easy

Visit the Python Cookbook: Functions Lab to test out this Shortcut in a live practice environment.

Problem

You would like to extend a closure with functions that allow the inner variables to be accessed and modified.

Solution

Normally, the inner variables of a closure are completely hidden to the outside world. However, you can provide access by writing accessor functions and attaching them to the closure as function attributes. For example:

def sample():
    n = 0
    # Closure function
    def func():
        print('n=', n)

    # Accessor methods for n
    def get_n():
        return n

    def set_n(value):
        nonlocal n
        n = value

    # Attach as function attributes
    func.get_n = get_n
    func.set_n = set_n
    return func

Here is an example of using this code:

>>> f = sample()
>>> f()
n= 0
>>> f.set_n(10)
>>> f()
n= 10
>>> f.get_n()
10
>>>

Discussion

There are two main features that make this Shortcut work. First, nonlocal declarations make it possible to write functions that change inner variables. Second, function attributes allow the accessor methods to be attached to the closure function in a straightforward manner where they work a lot like instance methods (even though no class is involved).

A slight extension to this Shortcut can be made to have closures emulate instances of a class. All you need to do is copy the inner functions over to the dictionary of an instance and return it. For example:

import ...