This module provides low-level primitives for working with multiple threads (also called light-weight processes or tasks) — multiple threads of control sharing their global data space. For synchronisation, simple locks (also called mutexes or binary semaphores) are provided.

When a thread specific error occurs a RuntimeError exception is raised.

Quick Usage Example

import _thread
import time

def th_func(delay, id):
    while True:
        print('Running thread %d' % id)

for i in range(2):
    _thread.start_new_thread(th_func, (i + 1, i))


_thread.start_new_thread(function, args[, kwargs])

Start a new thread and return its identifier. The thread executes the function with the argument list args (which must be a tuple). The optional kwargs argument specifies a dictionary of keyword arguments. When the function returns, the thread silently exits. When the function terminates with an unhandled exception, a stack trace is printed and then the thread exits (but other threads continue to run).

import _thread

def foo(arg):

_thread.start_new_thread(foo, (arg,))


Raise the SystemExit exception. When not caught, this will cause the thread to exit silently.


Return a new lock object. Methods of locks are described below. The lock is initially unlocked.


Return the thread identifier of the current thread. This is a nonzero integer. Its value has no direct meaning; it is intended as a magic cookie to be used e.g. to index a dictionary of thread-specific data. Thread identifiers may be recycled when a thread exits and another thread is created.


Return the thread stack size (in bytes) used when creating new threads. The optional size argument specifies the stack size to be used for subsequently created threads, and must be 0 (use platform or configured default) or a positive integer value of at least 4096 (4KiB). 4KiB is currently the minimum supported stack size value to guarantee sufficient stack space for the interpreter itself.



This is the type of lock objects.

class Lock

Used for synchronisation between threads


Lock objects have the following methods:

lock.acquire(waitflag=1, timeout=-1)

Without any optional argument, this method acquires the lock unconditionally, if necessary waiting until it is released by another thread (only one thread at a time can acquire a lock — that’s their reason for existence).

If the integer waitflag argument is present, the action depends on its value: if it is zero, the lock is only acquired if it can be acquired immediately without waiting, while if it is nonzero, the lock is acquired unconditionally as above.

If the floating-point timeout argument is present and positive, it specifies the maximum wait time in seconds before returning. A negative timeout argument specifies an unbounded wait. You cannot specify a timeout if waitflag is zero.

The return value is True if the lock is acquired successfully, False if not.


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


Return the status of the lock: True if it has been acquired by some thread, False if not.

In addition to these methods, lock objects can also be used via the with statement, e.g.:

import _thread

a_lock = _thread.allocate_lock()

with a_lock:
    print("a_lock is locked while this executes")