16.3. thread
— Multiple threads of control¶
Note
The thread
module has been renamed to _thread
in Python 3.
The 2to3 tool will automatically adapt imports when converting your
sources to Python 3; however, you should consider using the high-level
threading
module instead.
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:
-
exception
thread.
error
¶ Raised on thread-specific errors.
-
thread.
LockType
¶ 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).
-
thread.
interrupt_main
()¶ Raise a
KeyboardInterrupt
exception in the main thread. A subthread can use this function to interrupt the main thread.New in version 2.3.
-
thread.
exit
()¶ Raise the
SystemExit
exception. When not caught, this will cause the thread to exit silently.
-
thread.
allocate_lock
()¶ Return a new lock object. Methods of locks are described below. The lock is initially unlocked.
-
thread.
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.
-
thread.
stack_size
([size])¶ 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 size is not specified, 0 is used. If changing the thread stack size is unsupported, the
error
exception is raised. If the specified stack size is invalid, aValueError
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.New in version 2.5.
Lock objects have the following methods:
-
lock.
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). 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.
-
lock.
release
()¶ Releases the lock. The lock must have been acquired earlier, but not necessarily by the same thread.
-
lock.
locked
()¶ 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"
Caveats:
Threads interact strangely with interrupts: the
KeyboardInterrupt
exception will be received by an arbitrary thread. (When thesignal
module is available, interrupts always go to the main thread.)Calling
sys.exit()
or raising theSystemExit
exception is equivalent to callingthread.exit()
.It is not possible to interrupt the
acquire()
method on a lock — theKeyboardInterrupt
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.