threading.Timer()

Question

i have to write a program in network course that is something like selective repeat but a need a timer. after search in google i found that threading.Timer can help me, i wrote a simple program just for test how threading.Timer work that was this:

import threading

def hello():
    print "hello, world"

t = threading.Timer(10.0, hello)
t.start() 
print "Hi"
i=10
i=i+20
print i

this program run correctly. but when i try to define hello function in a way that give parameter like:

import threading

def hello(s):
    print s

h="hello world"
t = threading.Timer(10.0, hello(h))
t.start() 
print "Hi"
i=10
i=i+20
print i

the out put is :

hello world
Hi
30
Exception in thread Thread-1:
Traceback (most recent call last):
  File "/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/threading.py", line 522, in __bootstrap_inner
    self.run()
  File "/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/threading.py", line 726, in run
    self.function(*self.args, **self.kwargs)
TypeError: 'NoneType' object is not callable

i cant understand what is the problem! can any one help me?

Answer 1

You just need to put the arguments to hello into a separate item in the function call, like this,

t = threading.Timer(10.0, hello, [h])

This is a common approach in Python. Otherwise, when you use Timer(10.0, hello(h)) , the result of this function call is passed to Timer , which is None since hello doesn't make an explicit return.

Answer 2

An alternative is to use lambda if you want to use normal function parameters. Basically it tells the program that the argument is a function and not to be called on assignment.

t = threading.Timer(10.0, lambda: hello(h))