Please explain why i am getting a type error along with the expected output in the below decorator program.

Question

My code:
def decor(func):
print("============")
func()
print("============")
@decor
def print_text():
print("Hello world!")
print_text();

Output:
============
Hello world!
============

Traceback (most recent call last):
File "..\Playground\", line 11, in <module>
print_text();
TypeError: 'NoneType' object is not callable

Answer 1

Your decorator needs to return a function, but it's not returning anything, hence the 'TypeError: 'NoneType' object is not callable'. 

Try doing:

def decor(func):
def decorated():
print("============")
func()
print("============")
return decorated

@decor
def print_text():
print("Hello world!")

Answer 2

When we are doing

@decor
def print_text():
print("Hello world!")

What happens underneath the hood is

def print_text();
print("Hello world!")
print_text = decor(print_text)

This is why decor must return the internal function wrapper (in the case, decorated). Take a closer look at the third line

print_text = decor(print_text)

decor() takes print_text as a parameter, and returns the internal wrapper, which consist of the bars along with text. Where does it return to? It is returned back to print_text. So now, print_text actually stores the definition of decor's internal wrapper with the initial print_text definition.

Answer 3

@Hatsy Rei

Thank you for the solution.

I was trying to avoid the usage of extract function decorated(). It will work if we are calling the function like decor(print_text);

def decor(func):
print("============")
func()
print("============")
def print_text():
print("Hello world!")
decor(print_text);

But while using "@" we need to have a decorative function which should RETURN SOMETHING. So we need to use a nested function like decorated() inside decor().

def decor(func):
def decorated():
print("============")
func()
print("============")
return decorated

@decor
def print_text():
print("Hello world!")

Please let me know whether my understanding is correct or not.

Once again , thanks a lot for your quick response.

Answer 4

Got it. Thanks Hatsy.