Custom decorator is not allowing function to be called in main Python

Question

In a specific class I built which essentially acts as a Wrapper for a user-defined function, a user would only be using 2-3 main functions from my class aside from instantiation. I attempted to make my class decorator compatible so that the user could simply build his function and use the decorator instead of explicitly calling 2-3 functions from the class I built. I am now trying to test this.

Main:

from class_I_built import ClassIBuilt

def main():
    testfcn()


@ClassIBuilt('1', MessageConverter, timeout=10)
def testfcn():
    count = 0
    while count < 1000:
        count = count + 1

main()

class_I_built

import socket
from apb import APB
import config
import time

MCAST_GRP = config.MCAST_GRP
MCAST_PORT = config.MCAST_PORT


class ClassIBuilt(APB):
    processvar = 0

    def __init__(self, id, message_protocol, timeout=config.TIMEOUT):

        self.sock = 1

        self.p_id = p_id
        self.timeout = timeout
        self.message_protocol = message_protocol

    def __call__(self, decorator_function=1):
        self.publish_start('starting message')
        decorator_function(*args, **kwargs)
        self.publish_end('ending message')

    def connect(self):
        ttl = 2
        self.sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
        self.sock.setsockopt(socket.IPPROTO_IP, socket.IP_MULTICAST_TTL, ttl)

    def publish(self, message_type='alert', message=''):


        self.connect()
        s = self.message_protocol(message)
        try:
            self.sock.sendto(s, (MCAST_GRP, MCAST_PORT))
        finally:
            self.sock.close()

    def publish_start(self, message=''):
        print 'publishing start'
        self.publish("start", message)

    def publish_end(self, message=''):
        print 'publishing end'
        self.publish('end', message)

If I don't call testfcn() in main(), then the __call__ method is called and the start and end messages are published as normal. It should not work this way though, since it should only run testfcn() if I call testfcn() explicitly in main(). However, if I call testfcn() in main(), as shown above, then it will send the messages but I get a NoneType object is not callable error.

Output/stack trace:

publishing start
publishing end
Traceback (most recent call last):
  File "Main.py", line 13, in <module>
    main()
  File "Main.py", line 4, in main
    testfcn()
TypeError: 'NoneType' object is not callable

Why is the error occurring, and how can I structure my code so that it only runs if the function is called explicitly in main?

score 3 · Accepted Answer · answered Jul 01 '15 at 14:12

Your decorator line creates an instance of your class:

@ClassIBuilt('1', MessageConverter, timeout=10)

That instance is then used to do the actual decorating; you in effect do this:

decorator = ClassIBuilt('1', MessageConverter, timeout=10)
def testfcn(): # ...
testfcn = decorator(testfcn)

Because decorator is an instance of your class, the decorator(testfcn) call is handled by the ClassIBuilt.__call__() method, and that method returns nothing:

def __call__(self, decorator_function=1):
    self.publish_start('starting message')
    decorator_function(*args, **kwargs)
    self.publish_end('ending message')

No return statement means None is returned, so testfcn is set to None. You cannot call None, resulting in the exception you see.

Your __call__ should instead return a wrapper:

def __call__(self, decorator_function=1):
    def wrapper(*args, **kwargs):
        self.publish_start('starting message')
        decorator_function(*args, **kwargs)
        self.publish_end('ending message')
    return wrapper

Now testfcn is bound to the wrapper() function, which then calls the original decorated function.

Thanks. This seemed to fix both errors, but could you possibly explain why the testfcn = decorator(testfcn) line was still running even when it wasn't called in main? — Jeremy Fisher, Jul 01 '15 at 14:20
@JeremyFisher: I was illustrating what the decorator syntax *does*. The very act of putting the decorator before the `testfcn` function definition runs the decorator. — Martijn Pieters, Jul 01 '15 at 14:21

Custom decorator is not allowing function to be called in main Python

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