Closure with Arguments

There are different ways to use arguments in closure:

Argument for the outer function.
Argument for the inner function.

Outer Argument

Arguments for the outer function is used to enclose the values inside the function exactly once. This works like initialization:


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def start_count(start=1, step=1):
    counter = start

    def inner():
        nonlocal counter
        result = counter

        counter += step

        return result

    return inner

first_counter = start_count()
second_counter = start_count(13, 14)

print("First counter:")
print(first_counter())
print(first_counter())

print("Second counter:")
print(second_counter())
print(second_counter())

print("First counter:")
print(first_counter())
print(first_counter())

print("Second counter:")
print(second_counter())
print(second_counter())

The example above implements a counter function. The start_count() function takes two arguments: start and step, with default values of 1. The start value is assigned to the variable counter. The counter and step are then used inside the inner() function.

The arguments' values start (counter) and step are enclosed in the inner() function as parameters for the counter.

The variables first_counter and second_counter contain the inner() functions with enclosed parameters. These variables are functions that can be called without parameters.

Note

The variables first_counter and second_counter are independent counters: as an example, we can see how each of them has its own separate value and performs certain actions.

Inner Arguments

The inner() function is a returned function assigned to variables. The arguments that the inner() takes are used every time the inner() function is called via variables.


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def hero_hit(hero_name="Unknown"):
    def inner(monster="Unknown"):
        print("The", hero_name, "hero hit the", monster, "monster")
    
    return inner


john = hero_hit("John")  # use the outer argument
john("Goblin")  # use the inner argument

john("Dinosaur")  # inner("Goblin")
john()  # inner()
john("Dragon")  # inner("Dragon")

Arguments passed to the inner() function behave like regular function arguments and are not related to the closure.

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Section 2. Chapter 3