Example Decorator Error

Question

I'm trying to understand Python decorators and have written this code:

def hello_world(fn):
    print('hello world')
    fn()
    pass

@hello_world
def decorate():
    print('hello decoatrion')
    return

decorate()

I was aiming to print 'hello world' before 'hello decoration', but the output is the following:

hello world
hello decoatrion
Traceback (most recent call last):
  File "test_decortor.py", line 11, in <module>
    decorate()
TypeError: 'NoneType' object is not callable

Possible duplicate of [How can I make a chain of function decorators in Python?](http://stackoverflow.com/questions/739654/how-can-i-make-a-chain-of-function-decorators-in-python) — GingerPlusPlus, Jul 23 '16 at 13:13
Your decorator is returning `None`, not the decorated function. Check out [these examples](https://docs.python.org/3/whatsnew/2.4.html#pep-318-decorators-for-functions-and-methods), as well as [PEP 318 - Decorators for Functions, Methods and Classes](https://www.python.org/dev/peps/pep-0318/) itself. — martineau, Jul 23 '16 at 13:19

score 3 · Answer 1 · edited Sep 27 '17 at 05:58

3

The decorator must return the decorated function. You probably wanted something along these lines:

def hello_world(fn):
    def inner():
        print('hello world')
        fn()
    return inner

@hello_world
def decorate():
    print('hello decoatrion')
    return

decorate()
#output: hello world
#        hello decoatrion

edited Sep 27 '17 at 05:58

Graham

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answered Jul 23 '16 at 13:09

Aran-Fey

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score 3 · Accepted Answer · answered Jul 23 '16 at 13:13

Decorator syntax is shorthand for

decorated = decorate(decorated)

So if you have:

def hello_world(fn):
    print('hello world')
    fn()
    pass

def decorate():
    print('hello decoatrion')
    return

decorate = hello_world(decorate)

You should see what the problem is (also note that pass does nothing here).

def hello_world(fn):
    def says_hello():
        print('hello world')
        return fn()
    return says_hello

def decorate():
    print('hello decoration')

decorate = hello_world(decorate)

will do what you want. Or you could write it:

@hello_world
def decorate():
    print('hello decoration')

Documentation reference: https://docs.python.org/3/reference/compound_stmts.html#function-definitions — melpomene, Jul 23 '16 at 13:14

Example Decorator Error

2 Answers2