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/Doc/library/thread.rst

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  1:mod:`thread` --- Multiple threads of control
  2=============================================
  3
  4.. module:: thread
  5   :synopsis: Create multiple threads of control within one interpreter.
  6
  7.. note::
  8   The :mod:`thread` module has been renamed to :mod:`_thread` in Python 3.0.
  9   The :term:`2to3` tool will automatically adapt imports when converting your
 10   sources to 3.0; however, you should consider using the high-level
 11   :mod:`threading` module instead.
 12
 13
 14.. index::
 15   single: light-weight processes
 16   single: processes, light-weight
 17   single: binary semaphores
 18   single: semaphores, binary
 19
 20This module provides low-level primitives for working with multiple threads
 21(also called :dfn:`light-weight processes` or :dfn:`tasks`) --- multiple threads of
 22control sharing their global data space.  For synchronization, simple locks
 23(also called :dfn:`mutexes` or :dfn:`binary semaphores`) are provided.
 24The :mod:`threading` module provides an easier to use and higher-level
 25threading API built on top of this module.
 26
 27.. index::
 28   single: pthreads
 29   pair: threads; POSIX
 30
 31The module is optional.  It is supported on Windows, Linux, SGI IRIX, Solaris
 322.x, as well as on systems that have a POSIX thread (a.k.a. "pthread")
 33implementation.  For systems lacking the :mod:`thread` module, the
 34:mod:`dummy_thread` module is available. It duplicates this module's interface
 35and can be used as a drop-in replacement.
 36
 37It defines the following constant and functions:
 38
 39
 40.. exception:: error
 41
 42   Raised on thread-specific errors.
 43
 44
 45.. data:: LockType
 46
 47   This is the type of lock objects.
 48
 49
 50.. function:: start_new_thread(function, args[, kwargs])
 51
 52   Start a new thread and return its identifier.  The thread executes the function
 53   *function* with the argument list *args* (which must be a tuple).  The optional
 54   *kwargs* argument specifies a dictionary of keyword arguments. When the function
 55   returns, the thread silently exits.  When the function terminates with an
 56   unhandled exception, a stack trace is printed and then the thread exits (but
 57   other threads continue to run).
 58
 59
 60.. function:: interrupt_main()
 61
 62   Raise a :exc:`KeyboardInterrupt` exception in the main thread.  A subthread can
 63   use this function to interrupt the main thread.
 64
 65   .. versionadded:: 2.3
 66
 67
 68.. function:: exit()
 69
 70   Raise the :exc:`SystemExit` exception.  When not caught, this will cause the
 71   thread to exit silently.
 72
 73..
 74   function:: exit_prog(status)
 75
 76      Exit all threads and report the value of the integer argument
 77      *status* as the exit status of the entire program.
 78      **Caveat:** code in pending :keyword:`finally` clauses, in this thread
 79      or in other threads, is not executed.
 80
 81
 82.. function:: allocate_lock()
 83
 84   Return a new lock object.  Methods of locks are described below.  The lock is
 85   initially unlocked.
 86
 87
 88.. function:: get_ident()
 89
 90   Return the 'thread identifier' of the current thread.  This is a nonzero
 91   integer.  Its value has no direct meaning; it is intended as a magic cookie to
 92   be used e.g. to index a dictionary of thread-specific data.  Thread identifiers
 93   may be recycled when a thread exits and another thread is created.
 94
 95
 96.. function:: stack_size([size])
 97
 98   Return the thread stack size used when creating new threads.  The optional
 99   *size* argument specifies the stack size to be used for subsequently created
100   threads, and must be 0 (use platform or configured default) or a positive
101   integer value of at least 32,768 (32kB). If changing the thread stack size is
102   unsupported, the :exc:`error` exception is raised.  If the specified stack size is
103   invalid, a :exc:`ValueError` is raised and the stack size is unmodified.  32kB
104   is currently the minimum supported stack size value to guarantee sufficient
105   stack space for the interpreter itself.  Note that some platforms may have
106   particular restrictions on values for the stack size, such as requiring a
107   minimum stack size > 32kB or requiring allocation in multiples of the system
108   memory page size - platform documentation should be referred to for more
109   information (4kB pages are common; using multiples of 4096 for the stack size is
110   the suggested approach in the absence of more specific information).
111   Availability: Windows, systems with POSIX threads.
112
113   .. versionadded:: 2.5
114
115Lock objects have the following methods:
116
117
118.. method:: lock.acquire([waitflag])
119
120   Without the optional argument, this method acquires the lock unconditionally, if
121   necessary waiting until it is released by another thread (only one thread at a
122   time can acquire a lock --- that's their reason for existence).  If the integer
123   *waitflag* argument is present, the action depends on its value: if it is zero,
124   the lock is only acquired if it can be acquired immediately without waiting,
125   while if it is nonzero, the lock is acquired unconditionally as before.  The
126   return value is ``True`` if the lock is acquired successfully, ``False`` if not.
127
128
129.. method:: lock.release()
130
131   Releases the lock.  The lock must have been acquired earlier, but not
132   necessarily by the same thread.
133
134
135.. method:: lock.locked()
136
137   Return the status of the lock: ``True`` if it has been acquired by some thread,
138   ``False`` if not.
139
140In addition to these methods, lock objects can also be used via the
141:keyword:`with` statement, e.g.::
142
143   import thread
144
145   a_lock = thread.allocate_lock()
146
147   with a_lock:
148       print "a_lock is locked while this executes"
149
150**Caveats:**
151
152  .. index:: module: signal
153
154* Threads interact strangely with interrupts: the :exc:`KeyboardInterrupt`
155  exception will be received by an arbitrary thread.  (When the :mod:`signal`
156  module is available, interrupts always go to the main thread.)
157
158* Calling :func:`sys.exit` or raising the :exc:`SystemExit` exception is
159  equivalent to calling :func:`exit`.
160
161* Not all built-in functions that may block waiting for I/O allow other threads
162  to run.  (The most popular ones (:func:`time.sleep`, :meth:`file.read`,
163  :func:`select.select`) work as expected.)
164
165* It is not possible to interrupt the :meth:`acquire` method on a lock --- the
166  :exc:`KeyboardInterrupt` exception will happen after the lock has been acquired.
167
168  .. index:: pair: threads; IRIX
169
170* When the main thread exits, it is system defined whether the other threads
171  survive.  On SGI IRIX using the native thread implementation, they survive.  On
172  most other systems, they are killed without executing :keyword:`try` ...
173  :keyword:`finally` clauses or executing object destructors.
174
175* When the main thread exits, it does not do any of its usual cleanup (except
176  that :keyword:`try` ... :keyword:`finally` clauses are honored), and the
177  standard I/O files are not flushed.
178