Archive for June, 2008

Running Functions as Threads in Python

Saturday, June 28th, 2008

Suppose you have a function in some Python code that you want to run as a thread. How do you do it? The simplest way is via the thread module and its start_new_thread() method. This is illustrated in the following example.

import thread

def someFunc():
    print "someFunc was called"

thread.start_new_thread(someFunc, ())

This approach has a limitation in that once the start_new_thread() function is called, it is not possible to find out when the thread has finished or to wait for completion of the thread. This may be acceptable for many applications but may be too restrictive for others. Note that function parameters can also be passed in a tuple (use an empty tuple if there are no parameters) as the second argument to start_new_thread().

Python also provides the threading module which implements a layer on top of the thread module. The threading module provides, among other things, a Thread class which contains a run() method. Typical usage is to subclass the Thread class and override the run() method in the subclass to implement the desired functionality. The Thread class also provides start() and join() methods to control the starting of a thread and to provide a mechanism for waiting until the thread has finished execution (i.e. the end of run() method is reached).

Without sub-classing, it is possible to pass a function or other callable object to the Thread class constructor to specify the target that the run() method will call. This is illustrated below.

import threading

t1 = threading.Thread(target=someFunc)

This approach works well for providing a mechanism for waiting for the thread to complete. A drawback to this approach though is that it is not possible to pass any arguments to the function supplied as the thread’s target.

If waiting for thread completion and argument passing is required it is necessary to provide a subclass of Thread. The function to execute and its arguments can be passed to the subclass constructor. This is illustrated below:

import threading

class FuncThread(threading.Thread):
    def __init__(self, target, *args):
        self._target = target
        self._args = args

    def run(self):

# Example usage
def someOtherFunc(data, key):
    print "someOtherFunc was called : data=%s; key=%s" % (str(data), str(key))

t1 = FuncThread(someOtherFunc, [1,2], 6)