python decorator and wrapper function debug

Question

I'm studying the decorator and the wrapper by experimenting examples and I found the behavior of decorator that I can't understand.

def decorator_function(original_function):
    def wrapper_function():
        print "wrapper is executed before {}".format(original_function.__name__)
        return original_function() # Try it without ()
    return wrapper_function

@decorator_function 
def display():
    print("display function ran!")

when I run above function:

In [59]: display()
wrapper is executed before display
display function ran!

However, when I move the print function outside of the wrapper, it seems the wrapper doesn't run at all. (Probably it ran but I don't see the evidence.)

def decorator_function(original_function):
    def wrapper_function():
        #print "wrapper is executed before {}".format(original_function.__name__)
        return original_function() # Try it without ()
    print "wrapper is executed before {}".format(original_function.__name__)    
    return wrapper_function

@decorator_function 
def display():
    print("display function ran!")

Then I don't see "wrapper is executed before..." anymore. How come? What am I missing here?

In [63]: display()
display function ran!

larsks larsks · Accepted Answer · 2017-09-26T17:01:49

The decorator executes when your code loads, while the wrapper executes when your code runs. If I dump your second example into a file example.py and run it like this:

pythohn -i example.py

I see:

wrapper is executed before display
>>>

That was the decorator running. Now if I call display(), I see the output from that function:

>>> display()
display function ran!
>>>

I don't see the output from the decorator here because it already ran.

python decorator and wrapper function debug

1 Answers