Question

I’m wondering if it’s possible for a closure in Python to manipulate variables in its namespace. You might call this side-effects because the state is being changed outside the closure itself. I’d like to do something like this

def closureMaker():
  x = 0
  def closure():
    x+=1
    print x
  return closure

a = closureMaker()
a()
1
a()
2

Obviously what I hope to do is more complicated, but this example illustrates what I’m talking about.

Answer 1

You can’t do exactly that in Python 2.x, but you can use a trick to get the same effect: use a mutable object such as a list.

def closureMaker():
    x = [0]
    def closure():
        x[0] += 1
        print x[0]
    return closure

You can also make x an object with a named attribute, or a dictionary. This can be more readable than a list, especially if you have more than one such variable to modify.

In Python 3.x, you just need to add nonlocal x to your inner function. This causes assignments to x to go to the outer scope.