Python Low-level threading API

The '_thread' module in Python library provides a low-level interface for working with light-weight processes having multiple threads sharing a global data space. For synchronization, simple locks (also called mutexes or binary semaphores) are defined in this module. The 'threading' built-in module provides a higher-level threading API built on top of this module.


This module-level function is used to open a new thread in the current process. The function takes a function object as an argument. This function gets invoked on successful creation of the new thread. The span of this function corresponds to the lifespan of the thread. The thread may be blocked by calling sleep() function.

Following code is a simple example of thread mechanism using the _thread module.

import _thread
import time

def run( threadName):
   count = 0
   for i in range(1,6):
      print ( threadName, i )

_thread.start_new_thread( run, ("child", ) )

for i in range(1,6):
   print ( 'main', i )

The start_new_thread() function spawns a new thread which calls run function parallelly. There is a loop in the run() function as well as in the main thread of the program. Call to sleep() function in both results in overlapped execution as shown below: −

main 1
child 1
main 2
child 2
main 3
child 3
main 4
child 4
main 5
child 5

The inter-thread synchronization is achieved by using the Lock object. The allocate_lock() function returns lock object. Following methods are available for it


This method acquires the lock unconditionally until it is released by another thread. Only one thread at a time can acquire a lock. The return value is True if the lock is acquired successfully, False if not.


This method releases the lock. The lock must have been acquired earlier, but not necessarily by the same thread.

In the following example, two threads are declared. Each invokes run() function concurrently. One of them acquires the lock and proceeds to enter 'synchronized' function while other waits.

import _thread
import time

def run( threadName):

def synchronized(threadName):
   print (threadName,"has acquired lock")
   counter = 10
   while counter:
      print ('*', end='')
      counter = counter-1
      print('\n{} has released lock'.format( threadName))

lock = _thread.allocate_lock()

_thread.start_new_thread( run, ("t1", ) )
_thread.start_new_thread( run, ("t2", ) )


>>> t1 has acquired lock
t1 has released lock
t2 has acquired lock
t2 has released lock