Better illustration of Closures?

Question

I am learning Python using Dive Into Python 3 book. I like it, but I don't understand the example used to introduce Closures in Section 6.5.

I mean, I see how it works, and I think it's really cool. But I don't see any real benefit: it seems to me the same result could be achieved by simply reading in the rules file line by line in a loop, and doing search / replace for each line read.

Could someone help me to:

• either understand why using closures in this example improves the code (e.g., easier to maintain, extend, reuse, or debug?)

• or suggest a source of some other real-life code examples where closures really shine?

Answer 1

Decorators are an example of closures. For example,

def decorate(f):
    def wrapped_function():
        print("Function is being called")
        f()
        print("Function call is finished")
    return wrapped_function

@decorate
def my_function():
    print("Hello world")

my_function()

The function wrapped_function is a closure, because it retains access to the variables in its scope--in particular, the parameter f, the original function. Closures are what allow you to access it.

Closures also allow you to retain state across calls of a function, without having to resort to a class:

def make_counter():
    next_value = 0
    def return_next_value():
        nonlocal next_value
        val = next_value
        next_value += 1
        return val
    return return_next_value

my_first_counter = make_counter()
my_second_counter = make_counter()
print(my_first_counter())
print(my_second_counter())
print(my_first_counter())
print(my_second_counter())
print(my_first_counter())
print(my_second_counter())

Also, bound methods are technically closures (though they're probably implemented differently). Bound methods are class member functions with their class baked in:

import sys
w = sys.stdout.write
w("Hello\n")

w is essentially a closure with a reference to the sys.stdout object.

Finally, I haven't read that book, but a quick read of the chapter you linked and I'm very unimpressed--it's so horribly roundabout that it's useless as an explanation of closures.