Script to show how to write a manual decorator

manual_decorator.py

# Shows how to manually define and use a decorator

# A decorator is a function which expects another function as a parameter
def my_shiny_new_decorator(a_function_to_decorate):

    # Inside, the decorator defines a function on the fly: "the wrapper".
    # This function is going to be wrapped around the original function so it can execute
    # code before and after the original function
    def the_wrapper_around_the_original_function():

	# Put the code you want to execute BEFORE the original function is called
	print 'Before the function runs'

	# Call the original function (with parantheses)
	a_function_to_decorate()

	# Put the code you want to execute AFTER the original function is called
	print 'After the function runs'

    # At this point, "a_function_to_decorate" HAS NEVER BEEN executed. 
    # We return the "wrapper" function we just created
    # The wrapper contains the function and the code to execute before and after.
    # The decorator is then ready to use!
    return the_wrapper_around_the_original_function

# Now imagine you create a function you don't want to touch again
def a_stand_alone_function():
    print 'I am a stand alone function. Please do not modify me'

a_stand_alone_function()
print '\n\n'
# Outputs: I am a stand alone function. Please do not modify me

# Now we will decorate the a_stand_alone_function function.
# Just pass it to the decorator and call the returned function and it will 
# decorating the a_stand_alone_function function.
a_stand_alone_function = my_shiny_new_decorator(a_stand_alone_function)
a_stand_alone_function()

print '\n\n'

# Writing the same thing using the decorator syntax

@my_shiny_new_decorator
def another_stand_alone_function():
    print 'Leave me alone!'

# Now call the decorated function directly
another_stand_alone_function()