# 7.4 Built-in Module `thread`

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

The module is optional. It is supported on Windows NT and '95, SGI IRIX, Solaris 2.x, as well as on systems that have a POSIX thread (a.k.a. ``pthread'') implementation.

It defines the following constant and functions:

`error`
  Raised on thread-specific errors.

`start_new_thread` (func, arg)
  Start a new thread. The thread executes the function func with the argument list arg (which must be a tuple). 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).

`exit` ()
  This is a shorthand for `thread.exit_thread()`.

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

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

`get_ident` ()
  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.

Lock objects have the following methods:

`acquire` ([waitflag])
  Without the 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), and returns `None`. 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 before. If an argument is present, the return value is 1 if the lock is acquired successfully, 0 if not.

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

`locked` ()
  Return the status of the lock: 1 if it has been acquired by some thread, 0 if not.

Caveats:

- Threads interact strangely with interrupts: the `KeyboardInterrupt` exception will be received by an arbitrary thread. (When the `signal` module is available, interrupts always go to the main thread.)
- Calling `sys.exit()` or raising the `SystemExit` exception is equivalent to calling `thread.exit_thread()`.
- Not all built-in functions that may block waiting for I/O allow other threads to run. (The most popular ones (`sleep`, `read`, `select`) work as expected.)
- It is not possible to interrupt the `acquire()` method on a lock - the `KeyboardInterrupt` exception will happen after the lock has been acquired.
- When the main thread exits, it is system defined whether the other threads survive. On SGI IRIX using the native thread implementation, they survive. On most other systems, they are killed without executing ``try-finally'' clauses or executing object destructors.
- When the main thread exits, it doesn't do any of its usual cleanup (except that ``try-finally'' clauses are honored), and the standard I/O files are not flushed.