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(python2.1-lib.info)thread
Multiple threads of control
===========================
Create multiple threads of control within one interpreter.
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.
`LockType'
This is the type of lock objects.
`start_new_thread(function, args[, kwargs])'
Start a new thread. The thread executes the function 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).
`exit()'
Raise the `SystemExit' exception. When not caught, this will
cause the thread to exit silently.
`exit_thread()'
_This is deprecated in Python 1.5.2. Use `exit()'._ This is an
obsolete synonym for `exit()'.
`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 `exit()'.
* Not all built-in functions that may block waiting for I/O allow
other threads to run. (The most popular ones (`time.sleep()',
`FILE.read()', `select.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 does not do any of its usual cleanup
(except that `try' ... `finally' clauses are honored), and the
standard I/O files are not flushed.
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