_thread — Low-level threading API
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 synchronization, simple locks
(also called mutexes or binary semaphores) are provided.
The threading module provides an easier to use and higher-level
threading API built on top of this module.
The module is optional. It is supported on Windows, Linux, SGI IRIX, Solaris
2.x, as well as on systems that have a POSIX thread (a.k.a. “pthread”)
implementation. For systems lacking the _thread module, the
_dummy_thread module is available. It duplicates this module’s interface
and can be used as a drop-in replacement.
It defines the following constant and functions:
- Raised on thread-specific errors.
- This is the type of lock objects.
_thread.start_new_thread(function, args[, kwargs])
- Start a new thread and return its identifier. 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).
- Raise a KeyboardInterrupt exception in the main thread. A subthread can
use this function to interrupt the main thread.
- 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
- 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 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 32,768 (32kB). If changing the thread stack size is
unsupported, a ThreadError is raised. If the specified stack size is
invalid, a ValueError is raised and the stack size is unmodified. 32kB
is currently the minimum supported stack size value to guarantee sufficient
stack space for the interpreter itself. Note that some platforms may have
particular restrictions on values for the stack size, such as requiring a
minimum stack size > 32kB or requiring allocation in multiples of the system
memory page size - platform documentation should be referred to for more
information (4kB pages are common; using multiples of 4096 for the stack size is
the suggested approach in the absence of more specific information).
Availability: Windows, systems with POSIX threads.
Lock objects have the following methods:
- 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). 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. 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.:
a_lock = _thread.allocate_lock()
print("a_lock is locked while this executes")
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.