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threading.py
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"""Thread module emulating a subset of Java's threading model.""" import os as _os import sys as _sys import _thread import functools from time import monotonic as _time from _weakrefset import WeakSet from itertools import islice as _islice, count as _count try: from _collections import deque as _deque except ImportError: from collections import deque as _deque # Note regarding PEP 8 compliant names # This threading model was originally inspired by Java, and inherited # the convention of camelCase function and method names from that # language. Those original names are not in any imminent danger of # being deprecated (even for Py3k),so this module provides them as an # alias for the PEP 8 compliant names # Note that using the new PEP 8 compliant names facilitates substitution # with the multiprocessing module, which doesn't provide the old # Java inspired names. __all__ = ['get_ident', 'active_count', 'Condition', 'current_thread', 'enumerate', 'main_thread', 'TIMEOUT_MAX', 'Event', 'Lock', 'RLock', 'Semaphore', 'BoundedSemaphore', 'Thread', 'Barrier', 'BrokenBarrierError', 'Timer', 'ThreadError', 'setprofile', 'settrace', 'local', 'stack_size', 'excepthook', 'ExceptHookArgs'] # Rename some stuff so "from threading import *" is safe _start_new_thread = _thread.start_new_thread _allocate_lock = _thread.allocate_lock _set_sentinel = _thread._set_sentinel get_ident = _thread.get_ident try: get_native_id = _thread.get_native_id _HAVE_THREAD_NATIVE_ID = True __all__.append('get_native_id') except AttributeError: _HAVE_THREAD_NATIVE_ID = False ThreadError = _thread.error try: _CRLock = _thread.RLock except AttributeError: _CRLock = None TIMEOUT_MAX = _thread.TIMEOUT_MAX del _thread # Support for profile and trace hooks _profile_hook = None _trace_hook = None def setprofile(func): """Set a profile function for all threads started from the threading module. The func will be passed to sys.setprofile() for each thread, before its run() method is called. """ global _profile_hook _profile_hook = func def settrace(func): """Set a trace function for all threads started from the threading module. The func will be passed to sys.settrace() for each thread, before its run() method is called. """ global _trace_hook _trace_hook = func # Synchronization classes Lock = _allocate_lock def RLock(*args, **kwargs): """Factory function that returns a new reentrant lock. A reentrant lock must be released by the thread that acquired it. Once a thread has acquired a reentrant lock, the same thread may acquire it again without blocking; the thread must release it once for each time it has acquired it. """ if _CRLock is None: return _PyRLock(*args, **kwargs) return _CRLock(*args, **kwargs) class _RLock: """This class implements reentrant lock objects. A reentrant lock must be released by the thread that acquired it. Once a thread has acquired a reentrant lock, the same thread may acquire it again without blocking; the thread must release it once for each time it has acquired it. """ def __init__(self): self._block = _allocate_lock() self._owner = None self._count = 0 def __repr__(self): owner = self._owner try: owner = _active[owner].name except KeyError: pass return "<%s %s.%s object owner=%r count=%d at %s>" % ( "locked" if self._block.locked() else "unlocked", self.__class__.__module__, self.__class__.__qualname__, owner, self._count, hex(id(self)) ) def _at_fork_reinit(self): self._block._at_fork_reinit() self._owner = None self._count = 0 def acquire(self, blocking=True, timeout=-1): """Acquire a lock, blocking or non-blocking. When invoked without arguments: if this thread already owns the lock, increment the recursion level by one, and return immediately. Otherwise, if another thread owns the lock, block until the lock is unlocked. Once the lock is unlocked (not owned by any thread), then grab ownership, set the recursion level to one, and return. If more than one thread is blocked waiting until the lock is unlocked, only one at a time will be able to grab ownership of the lock. There is no return value in this case. When invoked with the blocking argument set to true, do the same thing as when called without arguments, and return true. When invoked with the blocking argument set to false, do not block. If a call without an argument would block, return false immediately; otherwise, do the same thing as when called without arguments, and return true. When invoked with the floating-point timeout argument set to a positive value, block for at most the number of seconds specified by timeout and as long as the lock cannot be acquired. Return true if the lock has been acquired, false if the timeout has elapsed. """ me = get_ident() if self._owner == me: self._count += 1 return 1 rc = self._block.acquire(blocking, timeout) if rc: self._owner = me self._count = 1 return rc __enter__ = acquire def release(self): """Release a lock, decrementing the recursion level. If after the decrement it is zero, reset the lock to unlocked (not owned by any thread), and if any other threads are blocked waiting for the lock to become unlocked, allow exactly one of them to proceed. If after the decrement the recursion level is still nonzero, the lock remains locked and owned by the calling thread. Only call this method when the calling thread owns the lock. A RuntimeError is raised if this method is called when the lock is unlocked. There is no return value. """ if self._owner != get_ident(): raise RuntimeError("cannot release un-acquired lock") self._count = count = self._count - 1 if not count: self._owner = None self._block.release() def __exit__(self, t, v, tb): self.release() # Internal methods used by condition variables def _acquire_restore(self, state): self._block.acquire() self._count, self._owner = state def _release_save(self): if self._count == 0: raise RuntimeError("cannot release un-acquired lock") count = self._count self._count = 0 owner = self._owner self._owner = None self._block.release() return (count, owner) def _is_owned(self): return self._owner == get_ident() _PyRLock = _RLock class Condition: """Class that implements a condition variable. A condition variable allows one or more threads to wait until they are notified by another thread. If the lock argument is given and not None, it must be a Lock or RLock object, and it is used as the underlying lock. Otherwise, a new RLock object is created and used as the underlying lock. """ def __init__(self, lock=None): if lock is None: lock = RLock() self._lock = lock # Export the lock's acquire() and release() methods self.acquire = lock.acquire self.release = lock.release # If the lock defines _release_save() and/or _acquire_restore(), # these override the default implementations (which just call # release() and acquire() on the lock). Ditto for _is_owned(). try: self._release_save = lock._release_save except AttributeError: pass try: self._acquire_restore = lock._acquire_restore except AttributeError: pass try: self._is_owned = lock._is_owned except AttributeError: pass self._waiters = _deque() def _at_fork_reinit(self): self._lock._at_fork_reinit() self._waiters.clear() def __enter__(self): return self._lock.__enter__() def __exit__(self, *args): return self._lock.__exit__(*args) def __repr__(self): return "<Condition(%s, %d)>" % (self._lock, len(self._waiters)) def _release_save(self): self._lock.release() # No state to save def _acquire_restore(self, x): self._lock.acquire() # Ignore saved state def _is_owned(self): # Return True if lock is owned by current_thread. # This method is called only if _lock doesn't have _is_owned(). if self._lock.acquire(False): self._lock.release() return False else: return True def wait(self, timeout=None): """Wait until notified or until a timeout occurs. If the calling thread has not acquired the lock when this method is called, a RuntimeError is raised. This method releases the underlying lock, and then blocks until it is awakened by a notify() or notify_all() call for the same condition variable in another thread, or until the optional timeout occurs. Once awakened or timed out, it re-acquires the lock and returns. When the timeout argument is present and not None, it should be a floating point number specifying a timeout for the operation in seconds (or fractions thereof). When the underlying lock is an RLock, it is not released using its release() method, since this may not actually unlock the lock when it was acquired multiple times recursively. Instead, an internal interface of the RLock class is used, which really unlocks it even when it has been recursively acquired several times. Another internal interface is then used to restore the recursion level when the lock is reacquired. """ if not self._is_owned(): raise RuntimeError("cannot wait on un-acquired lock") waiter = _allocate_lock() waiter.acquire() self._waiters.append(waiter) saved_state = self._release_save() gotit = False try: # restore state no matter what (e.g., KeyboardInterrupt) if timeout is None: waiter.acquire() gotit = True else: if timeout > 0: gotit = waiter.acquire(True, timeout) else: gotit = waiter.acquire(False) return gotit finally: self._acquire_restore(saved_state) if not gotit: try: self._waiters.remove(waiter) except ValueError: pass def wait_for(self, predicate, timeout=None): """Wait until a condition evaluates to True. predicate should be a callable which result will be interpreted as a boolean value. A timeout may be provided giving the maximum time to wait. """ endtime = None waittime = timeout result = predicate() while not result: if waittime is not None: if endtime is None: endtime = _time() + waittime else: waittime = endtime - _time() if waittime <= 0: break self.wait(waittime) result = predicate() return result def notify(self, n=1): """Wake up one or more threads waiting on this condition, if any. If the calling thread has not acquired the lock when this method is called, a RuntimeError is raised. This method wakes up at most n of the threads waiting for the condition variable; it is a no-op if no threads are waiting. """ if not self._is_owned(): raise RuntimeError("cannot notify on un-acquired lock") all_waiters = self._waiters waiters_to_notify = _deque(_islice(all_waiters, n)) if not waiters_to_notify: return for waiter in waiters_to_notify: waiter.release() try: all_waiters.remove(waiter) except ValueError: pass def notify_all(self): """Wake up all threads waiting on this condition. If the calling thread has not acquired the lock when this method is called, a RuntimeError is raised. """ self.notify(len(self._waiters)) notifyAll = notify_all class Semaphore: """This class implements semaphore objects. Semaphores manage a counter representing the number of release() calls minus the number of acquire() calls, plus an initial value. The acquire() method blocks if necessary until it can return without making the counter negative. If not given, value defaults to 1. """ # After Tim Peters' semaphore class, but not quite the same (no maximum) def __init__(self, value=1): if value < 0: raise ValueError("semaphore initial value must be >= 0") self._cond = Condition(Lock()) self._value = value def acquire(self, blocking=True, timeout=None): """Acquire a semaphore, decrementing the internal counter by one. When invoked without arguments: if the internal counter is larger than zero on entry, decrement it by one and return immediately. If it is zero on entry, block, waiting until some other thread has called release() to make it larger than zero. This is done with proper interlocking so that if multiple acquire() calls are blocked, release() will wake exactly one of them up. The implementation may pick one at random, so the order in which blocked threads are awakened should not be relied on. There is no return value in this case. When invoked with blocking set to true, do the same thing as when called without arguments, and return true. When invoked with blocking set to false, do not block. If a call without an argument would block, return false immediately; otherwise, do the same thing as when called without arguments, and return true. When invoked with a timeout other than None, it will block for at most timeout seconds. If acquire does not complete successfully in that interval, return false. Return true otherwise. """ if not blocking and timeout is not None: raise ValueError("can't specify timeout for non-blocking acquire") rc = False endtime = None with self._cond: while self._value == 0: if not blocking: break if timeout is not None: if endtime is None: endtime = _time() + timeout else: timeout = endtime - _time() if timeout <= 0: break self._cond.wait(timeout) else: self._value -= 1 rc = True return rc __enter__ = acquire def release(self, n=1): """Release a semaphore, incrementing the internal counter by one or more. When the counter is zero on entry and another thread is waiting for it to become larger than zero again, wake up that thread. """ if n < 1: raise ValueError('n must be one or more') with self._cond: self._value += n for i in range(n): self._cond.notify() def __exit__(self, t, v, tb): self.release() class BoundedSemaphore(Semaphore): """Implements a bounded semaphore. A bounded semaphore checks to make sure its current value doesn't exceed its initial value. If it does, ValueError is raised. In most situations semaphores are used to guard resources with limited capacity. If the semaphore is released too many times it's a sign of a bug. If not given, value defaults to 1. Like regular semaphores, bounded semaphores manage a counter representing the number of release() calls minus the number of acquire() calls, plus an initial value. The acquire() method blocks if necessary until it can return without making the counter negative. If not given, value defaults to 1. """ def __init__(self, value=1): Semaphore.__init__(self, value) self._initial_value = value def release(self, n=1): """Release a semaphore, incrementing the internal counter by one or more. When the counter is zero on entry and another thread is waiting for it to become larger than zero again, wake up that thread. If the number of releases exceeds the number of acquires, raise a ValueError. """ if n < 1: raise ValueError('n must be one or more') with self._cond: if self._value + n > self._initial_value: raise ValueError("Semaphore released too many times") self._value += n for i in range(n): self._cond.notify() class Event: """Class implementing event objects. Events manage a flag that can be set to true with the set() method and reset to false with the clear() method. The wait() method blocks until the flag is true. The flag is initially false. """ # After Tim Peters' event class (without is_posted()) def __init__(self): self._cond = Condition(Lock()) self._flag = False def _at_fork_reinit(self): # Private method called by Thread._reset_internal_locks() self._cond._at_fork_reinit() def is_set(self): """Return true if and only if the internal flag is true.""" return self._flag isSet = is_set def set(self): """Set the internal flag to true. All threads waiting for it to become true are awakened. Threads that call wait() once the flag is true will not block at all. """ with self._cond: self._flag = True self._cond.notify_all() def clear(self): """Reset the internal flag to false. Subsequently, threads calling wait() will block until set() is called to set the internal flag to true again. """ with self._cond: self._flag = False def wait(self, timeout=None): """Block until the internal flag is true. If the internal flag is true on entry, return immediately. Otherwise, block until another thread calls set() to set the flag to true, or until the optional timeout occurs. When the timeout argument is present and not None, it should be a floating point number specifying a timeout for the operation in seconds (or fractions thereof). This method returns the internal flag on exit, so it will always return True except if a timeout is given and the operation times out. """ with self._cond: signaled = self._flag if not signaled: signaled = self._cond.wait(timeout) return signaled # A barrier class. Inspired in part by the pthread_barrier_* api and # the CyclicBarrier class from Java. See # http://sourceware.org/pthreads-win32/manual/pthread_barrier_init.html and # http://java.sun.com/j2se/1.5.0/docs/api/java/util/concurrent/ # CyclicBarrier.html # for information. # We maintain two main states, 'filling' and 'draining' enabling the barrier # to be cyclic. Threads are not allowed into it until it has fully drained # since the previous cycle. In addition, a 'resetting' state exists which is # similar to 'draining' except that threads leave with a BrokenBarrierError, # and a 'broken' state in which all threads get the exception. class Barrier: """Implements a Barrier. Useful for synchronizing a fixed number of threads at known synchronization points. Threads block on 'wait()' and are simultaneously awoken once they have all made that call. """ def __init__(self, parties, action=None, timeout=None): """Create a barrier, initialised to 'parties' threads. 'action' is a callable which, when supplied, will be called by one of the threads after they have all entered the barrier and just prior to releasing them all. If a 'timeout' is provided, it is used as the default for all subsequent 'wait()' calls. """ self._cond = Condition(Lock()) self._action = action self._timeout = timeout self._parties = parties self._state = 0 #0 filling, 1, draining, -1 resetting, -2 broken self._count = 0 def wait(self, timeout=None): """Wait for the barrier. When the specified number of threads have started waiting, they are all simultaneously awoken. If an 'action' was provided for the barrier, one of the threads will have executed that callback prior to returning. Returns an individual index number from 0 to 'parties-1'. """ if timeout is None: timeout = self._timeout with self._cond: self._enter() # Block while the barrier drains. index = self._count self._count += 1 try: if index + 1 == self._parties: # We release the barrier self._release() else: # We wait until someone releases us self._wait(timeout) return index finally: self._count -= 1 # Wake up any threads waiting for barrier to drain. self._exit() # Block until the barrier is ready for us, or raise an exception # if it is broken. def _enter(self): while self._state in (-1, 1): # It is draining or resetting, wait until done self._cond.wait() #see if the barrier is in a broken state if self._state < 0: raise BrokenBarrierError assert self._state == 0 # Optionally run the 'action' and release the threads waiting # in the barrier. def _release(self): try: if self._action: self._action() # enter draining state self._state = 1 self._cond.notify_all() except: #an exception during the _action handler. Break and reraise self._break() raise # Wait in the barrier until we are released. Raise an exception # if the barrier is reset or broken. def _wait(self, timeout): if not self._cond.wait_for(lambda : self._state != 0, timeout): #timed out. Break the barrier self._break() raise BrokenBarrierError if self._state < 0: raise BrokenBarrierError assert self._state == 1 # If we are the last thread to exit the barrier, signal any threads # waiting for the barrier to drain. def _exit(self): if self._count == 0: if self._state in (-1, 1): #resetting or draining self._state = 0 self._cond.notify_all() def reset(self): """Reset the barrier to the initial state. Any threads currently waiting will get the BrokenBarrier exception raised. """ with self._cond: if self._count > 0: if self._state == 0: #reset the barrier, waking up threads self._state = -1 elif self._state == -2: #was broken, set it to reset state #which clears when the last thread exits self._state = -1 else: self._state = 0 self._cond.notify_all() def abort(self): """Place the barrier into a 'broken' state. Useful in case of error. Any currently waiting threads and threads attempting to 'wait()' will have BrokenBarrierError raised. """ with self._cond: self._break() def _break(self): # An internal error was detected. The barrier is set to # a broken state all parties awakened. self._state = -2 self._cond.notify_all() @property def parties(self): """Return the number of threads required to trip the barrier.""" return self._parties @property def n_waiting(self): """Return the number of threads currently waiting at the barrier.""" # We don't need synchronization here since this is an ephemeral result # anyway. It returns the correct value in the steady state. if self._state == 0: return self._count return 0 @property def broken(self): """Return True if the barrier is in a broken state.""" return self._state == -2 # exception raised by the Barrier class class BrokenBarrierError(RuntimeError): pass # Helper to generate new thread names _counter = _count().__next__ _counter() # Consume 0 so first non-main thread has id 1. def _newname(template="Thread-%d"): return template % _counter() # Active thread administration. # # bpo-44422: Use a reentrant lock to allow reentrant calls to functions like # threading.enumerate(). _active_limbo_lock = RLock() _active = {} # maps thread id to Thread object _limbo = {} _dangling = WeakSet() # Set of Thread._tstate_lock locks of non-daemon threads used by _shutdown() # to wait until all Python thread states get deleted: # see Thread._set_tstate_lock(). _shutdown_locks_lock = _allocate_lock() _shutdown_locks = set() def _maintain_shutdown_locks(): """ Drop any shutdown locks that don't correspond to running threads anymore. Calling this from time to time avoids an ever-growing _shutdown_locks set when Thread objects are not joined explicitly. See bpo-37788. This must be called with _shutdown_locks_lock acquired. """ # If a lock was released, the corresponding thread has exited to_remove = [lock for lock in _shutdown_locks if not lock.locked()] _shutdown_locks.difference_update(to_remove) # Main class for threads class Thread: """A class that represents a thread of control. This class can be safely subclassed in a limited fashion. There are two ways to specify the activity: by passing a callable object to the constructor, or by overriding the run() method in a subclass. """ _initialized = False def __init__(self, group=None, target=None, name=None, args=(), kwargs=None, *, daemon=None): """This constructor should always be called with keyword arguments. Arguments are: *group* should be None; reserved for future extension when a ThreadGroup class is implemented. *target* is the callable object to be invoked by the run() method. Defaults to None, meaning nothing is called. *name* is the thread name. By default, a unique name is constructed of the form "Thread-N" where N is a small decimal number. *args* is the argument tuple for the target invocation. Defaults to (). *kwargs* is a dictionary of keyword arguments for the target invocation. Defaults to {}. If a subclass overrides the constructor, it must make sure to invoke the base class constructor (Thread.__init__()) before doing anything else to the thread. """ assert group is None, "group argument must be None for now" if kwargs is None: kwargs = {} self._target = target self._name = str(name or _newname()) self._args = args self._kwargs = kwargs if daemon is not None: self._daemonic = daemon else: self._daemonic = current_thread().daemon self._ident = None if _HAVE_THREAD_NATIVE_ID: self._native_id = None self._tstate_lock = None self._started = Event() self._is_stopped = False self._initialized = True # Copy of sys.stderr used by self._invoke_excepthook() self._stderr = _sys.stderr self._invoke_excepthook = _make_invoke_excepthook() # For debugging and _after_fork() _dangling.add(self) def _reset_internal_locks(self, is_alive): # private! Called by _after_fork() to reset our internal locks as # they may be in an invalid state leading to a deadlock or crash. self._started._at_fork_reinit() if is_alive: # bpo-42350: If the fork happens when the thread is already stopped # (ex: after threading._shutdown() has been called), _tstate_lock # is None. Do nothing in this case. if self._tstate_lock is not None: self._tstate_lock._at_fork_reinit() self._tstate_lock.acquire() else: # The thread isn't alive after fork: it doesn't have a tstate # anymore. self._is_stopped = True self._tstate_lock = None def __repr__(self): assert self._initialized, "Thread.__init__() was not called" status = "initial" if self._started.is_set(): status = "started" self.is_alive() # easy way to get ._is_stopped set when appropriate if self._is_stopped: status = "stopped" if self._daemonic: status += " daemon" if self._ident is not None: status += " %s" % self._ident return "<%s(%s, %s)>" % (self.__class__.__name__, self._name, status) def start(self): """Start the thread's activity. It must be called at most once per thread object. It arranges for the object's run() method to be invoked in a separate thread of control. This method will raise a RuntimeError if called more than once on the same thread object. """ if not self._initialized: raise RuntimeError("thread.__init__() not called") if self._started.is_set(): raise RuntimeError("threads can only be started once") with _active_limbo_lock: _limbo[self] = self try: _start_new_thread(self._bootstrap, ()) except Exception: with _active_limbo_lock: del _limbo[self] raise self._started.wait() def run(self): """Method representing the thread's activity. You may override this method in a subclass. The standard run() method invokes the callable object passed to the object's constructor as the target argument, if any, with sequential and keyword arguments taken from the args and kwargs arguments, respectively. """ try: if self._target: self._target(*self._args, **self._kwargs) finally: # Avoid a refcycle if the thread is running a function with # an argument that has a member that points to the thread. del self._target, self._args, self._kwargs def _bootstrap(self): # Wrapper around the real bootstrap code that ignores # exceptions during interpreter cleanup. Those typically # happen when a daemon thread wakes up at an unfortunate # moment, finds the world around it destroyed, and raises some # random exception *** while trying to report the exception in # _bootstrap_inner() below ***. Those random exceptions # don't help anybody, and they confuse users, so we suppress # them. We suppress them only when it appears that the world # indeed has already been destroyed, so that exceptions in # _bootstrap_inner() during normal business hours are properly # reported. Also, we only suppress them for daemonic threads; # if a non-daemonic encounters this, something else is wrong. try: self._bootstrap_inner() except: if self._daemonic and _sys is None: return raise def _set_ident(self): self._ident = get_ident() if _HAVE_THREAD_NATIVE_ID: def _set_native_id(self): self._native_id = get_native_id() def _set_tstate_lock(self): """ Set a lock object which will be released by the interpreter when the underlying thread state (see pystate.h) gets deleted. """ self._tstate_lock = _set_sentinel() self._tstate_lock.acquire() if not self.daemon: with _shutdown_locks_lock: _maintain_shutdown_locks() _shutdown_locks.add(self._tstate_lock) def _bootstrap_inner(self): try: self._set_ident() self._set_tstate_lock() if _HAVE_THREAD_NATIVE_ID: self._set_native_id() self._started.set() with _active_limbo_lock: _active[self._ident] = self del _limbo[self] if _trace_hook: _sys.settrace(_trace_hook) if _profile_hook: _sys.setprofile(_profile_hook) try: self.run() except: self._invoke_excepthook(self) finally: with _active_limbo_lock: try: # We don't call self._delete() because it also # grabs _active_limbo_lock. del _active[get_ident()] except: pass def _stop(self): # After calling ._stop(), .is_alive() returns False and .join() returns # immediately. ._tstate_lock must be released before calling ._stop(). # # Normal case: C code at the end of the thread's life # (release_sentinel in _threadmodule.c) releases ._tstate_lock, and # that's detected by our ._wait_for_tstate_lock(), called by .join() # and .is_alive(). Any number of threads _may_ call ._stop() # simultaneously (for example, if multiple threads are blocked in # .join() calls), and they're not serialized. That's harmless - # they'll just make redundant rebindings of ._is_stopped and # ._tstate_lock. Obscure: we rebind ._tstate_lock last so that the # "assert self._is_stopped" in ._wait_for_tstate_lock() always works # (the assert is executed only if ._tstate_lock is None). # # Special case: _main_thread releases ._tstate_lock via this # module's _shutdown() function. lock = self._tstate_lock if lock is not None: assert not lock.locked() self._is_stopped = True self._tstate_lock = None if not self.daemon: with _shutdown_locks_lock: # Remove our lock and other released locks from _shutdown_locks _maintain_shutdown_locks() def _delete(self): "Remove current thread from the dict of currently running threads." with _active_limbo_lock: del _active[get_ident()] # There must not be any python code between the previous line # and after the lock is released. Otherwise a tracing function # could try to acquire the lock again in the same thread, (in # current_thread()), and would block. def join(self, timeout=None): """Wait until the thread terminates. This blocks the calling thread until the thread whose join() method is called terminates -- either normally or through an unhandled exception or until the optional timeout occurs. When the timeout argument is present and not None, it should be a floating point number specifying a timeout for the operation in seconds (or fractions thereof). As join() always returns None, you must call is_alive() after join() to decide whether a timeout happened -- if the thread is still alive, the join() call timed out. When the timeout argument is not present or None, the operation will block until the thread terminates. A thread can be join()ed many times. join() raises a RuntimeError if an attempt is made to join the current thread as that would cause a deadlock. It is also an error to join() a thread before it has been started and attempts to do so raises the same exception. """ if not self._initialized: raise RuntimeError("Thread.__init__() not called") if not self._started.is_set(): raise RuntimeError("cannot join thread before it is started") if self is current_thread(): raise RuntimeError("cannot join current thread") if timeout is None: self._wait_for_tstate_lock() else: # the behavior of a negative timeout isn't documented, but # historically .join(timeout=x) for x<0 has acted as if timeout=0 self._wait_for_tstate_lock(timeout=max(timeout, 0)) def _wait_for_tstate_lock(self, block=True, timeout=-1): # Issue #18808: wait for the thread state to be gone. # At the end of the thread's life, after all knowledge of the thread # is removed from C data structures, C code releases our _tstate_lock. # This method passes its arguments to _tstate_lock.acquire(). # If the lock is acquired, the C code is done, and self._stop() is # called. That sets ._is_stopped to True, and ._tstate_lock to None. lock = self._tstate_lock if lock is None: # already determined that the C code is done assert self._is_stopped elif lock.acquire(block, timeout): lock.release() self._stop() @property def name(self): """A string used for identification purposes only. It has no semantics. Multiple threads may be given the same name. The initial name is set by the constructor. """ assert self._initialized, "Thread.__init__() not called" return self._name @name.setter def name(self, name): assert self._initialized, "Thread.__init__() not called" self._name = str(name) @property def ident(self): """Thread identifier of this thread or None if it has not been started. This is a nonzero integer. See the get_ident() function. Thread identifiers may be recycled when a thread exits and another thread is created. The identifier is available even after the thread has exited. """ assert self._initialized, "Thread.__init__() not called" return self._ident if _HAVE_THREAD_NATIVE_ID: @property def native_id(self): """Native integral thread ID of this thread, or None if it has not been started. This is a non-negative integer. See the get_native_id() function. This represents the Thread ID as reported by the kernel. """ assert self._initialized, "Thread.__init__() not called" return self._native_id def is_alive(self): """Return whether the thread is alive. This method returns True just before the run() method starts until just after the run() method terminates. See also the module function enumerate(). """ assert self._initialized, "Thread.__init__() not called" if self._is_stopped or not self._started.is_set(): return False self._wait_for_tstate_lock(False) return not self._is_stopped @property def daemon(self): """A boolean value indicating whether this thread is a daemon thread. This must be set before start() is called, otherwise RuntimeError is raised. Its initial value is inherited from the creating thread; the main thread is not a daemon thread and therefore all threads created in the main thread default to daemon = False. The entire Python program exits when only daemon threads are left. """ assert self._initialized, "Thread.__init__() not called" return self._daemonic @daemon.setter def daemon(self, daemonic): if not self._initialized: raise RuntimeError("Thread.__init__() not called") if self._started.is_set(): raise RuntimeError("cannot set daemon status of active thread") self._daemonic = daemonic def isDaemon(self): return self.daemon def setDaemon(self, daemonic): self.daemon = daemonic def getName(self): return self.name def setName(self, name): self.name = name try: from _thread import (_excepthook as excepthook, _ExceptHookArgs as ExceptHookArgs) except ImportError: # Simple Python implementation if _thread._excepthook() is not available from traceback import print_exception as _print_exception from collections import namedtuple _ExceptHookArgs = namedtuple( 'ExceptHookArgs', 'exc_type exc_value exc_traceback thread') def ExceptHookArgs(args): return _ExceptHookArgs(*args) def excepthook(args, /): """ Handle uncaught Thread.run() exception. """ if args.exc_type == SystemExit: # silently ignore SystemExit return if _sys is not None and _sys.stderr is not None: stderr = _sys.stderr elif args.thread is not None: stderr = args.thread._stderr if stderr is None: # do nothing if sys.stderr is None and sys.stderr was None # when the thread was created return else: # do nothing if sys.stderr is None and args.thread is None return if args.thread is not None: name = args.thread.name else: name = get_ident() print(f"Exception in thread {name}:", file=stderr, flush=True) _print_exception(args.exc_type, args.exc_value, args.exc_traceback, file=stderr) stderr.flush() def _make_invoke_excepthook(): # Create a local namespace to ensure that variables remain alive # when _invoke_excepthook() is called, even if it is called late during # Python shutdown. It is mostly needed for daemon threads. old_excepthook = excepthook old_sys_excepthook = _sys.excepthook if old_excepthook is None: raise RuntimeError("threading.excepthook is None") if old_sys_excepthook is None: raise RuntimeError("sys.excepthook is None") sys_exc_info = _sys.exc_info local_print = print local_sys = _sys def invoke_excepthook(thread): global excepthook try: hook = excepthook if hook is None: hook = old_excepthook args = ExceptHookArgs([*sys_exc_info(), thread]) hook(args) except Exception as exc: exc.__suppress_context__ = True del exc if local_sys is not None and local_sys.stderr is not None: stderr = local_sys.stderr else: stderr = thread._stderr local_print("Exception in threading.excepthook:", file=stderr, flush=True) if local_sys is not None and local_sys.excepthook is not None: sys_excepthook = local_sys.excepthook else: sys_excepthook = old_sys_excepthook sys_excepthook(*sys_exc_info()) finally: # Break reference cycle (exception stored in a variable) args = None return invoke_excepthook # The timer class was contributed by Itamar Shtull-Trauring class Timer(Thread): """Call a function after a specified number of seconds: t = Timer(30.0, f, args=None, kwargs=None) t.start() t.cancel() # stop the timer's action if it's still waiting """ def __init__(self, interval, function, args=None, kwargs=None): Thread.__init__(self) self.interval = interval self.function = function self.args = args if args is not None else [] self.kwargs = kwargs if kwargs is not None else {} self.finished = Event() def cancel(self): """Stop the timer if it hasn't finished yet.""" self.finished.set() def run(self): self.finished.wait(self.interval) if not self.finished.is_set(): self.function(*self.args, **self.kwargs) self.finished.set() # Special thread class to represent the main thread class _MainThread(Thread): def __init__(self): Thread.__init__(self, name="MainThread", daemon=False) self._set_tstate_lock() self._started.set() self._set_ident() if _HAVE_THREAD_NATIVE_ID: self._set_native_id() with _active_limbo_lock: _active[self._ident] = self # Dummy thread class to represent threads not started here. # These aren't garbage collected when they die, nor can they be waited for. # If they invoke anything in threading.py that calls current_thread(), they # leave an entry in the _active dict forever after. # Their purpose is to return *something* from current_thread(). # They are marked as daemon threads so we won't wait for them # when we exit (conform previous semantics). class _DummyThread(Thread): def __init__(self): Thread.__init__(self, name=_newname("Dummy-%d"), daemon=True) self._started.set() self._set_ident() if _HAVE_THREAD_NATIVE_ID: self._set_native_id() with _active_limbo_lock: _active[self._ident] = self def _stop(self): pass def is_alive(self): assert not self._is_stopped and self._started.is_set() return True def join(self, timeout=None): assert False, "cannot join a dummy thread" # Global API functions def current_thread(): """Return the current Thread object, corresponding to the caller's thread of control. If the caller's thread of control was not created through the threading module, a dummy thread object with limited functionality is returned. """ try: return _active[get_ident()] except KeyError: return _DummyThread() currentThread = current_thread def active_count(): """Return the number of Thread objects currently alive. The returned count is equal to the length of the list returned by enumerate(). """ with _active_limbo_lock: return len(_active) + len(_limbo) activeCount = active_count def _enumerate(): # Same as enumerate(), but without the lock. Internal use only. return list(_active.values()) + list(_limbo.values()) def enumerate(): """Return a list of all Thread objects currently alive. The list includes daemonic threads, dummy thread objects created by current_thread(), and the main thread. It excludes terminated threads and threads that have not yet been started. """ with _active_limbo_lock: return list(_active.values()) + list(_limbo.values()) _threading_atexits = [] _SHUTTING_DOWN = False def _register_atexit(func, *arg, **kwargs): """CPython internal: register *func* to be called before joining threads. The registered *func* is called with its arguments just before all non-daemon threads are joined in `_shutdown()`. It provides a similar purpose to `atexit.register()`, but its functions are called prior to threading shutdown instead of interpreter shutdown. For similarity to atexit, the registered functions are called in reverse. """ if _SHUTTING_DOWN: raise RuntimeError("can't register atexit after shutdown") call = functools.partial(func, *arg, **kwargs) _threading_atexits.append(call) from _thread import stack_size # Create the main thread object, # and make it available for the interpreter # (Py_Main) as threading._shutdown. _main_thread = _MainThread() def _shutdown(): """ Wait until the Python thread state of all non-daemon threads get deleted. """ # Obscure: other threads may be waiting to join _main_thread. That's # dubious, but some code does it. We can't wait for C code to release # the main thread's tstate_lock - that won't happen until the interpreter # is nearly dead. So we release it here. Note that just calling _stop() # isn't enough: other threads may already be waiting on _tstate_lock. if _main_thread._is_stopped: # _shutdown() was already called return global _SHUTTING_DOWN _SHUTTING_DOWN = True # Main thread tlock = _main_thread._tstate_lock # The main thread isn't finished yet, so its thread state lock can't have # been released. assert tlock is not None assert tlock.locked() tlock.release() _main_thread._stop() # Call registered threading atexit functions before threads are joined. # Order is reversed, similar to atexit. for atexit_call in reversed(_threading_atexits): atexit_call() # Join all non-deamon threads while True: with _shutdown_locks_lock: locks = list(_shutdown_locks) _shutdown_locks.clear() if not locks: break for lock in locks: # mimick Thread.join() lock.acquire() lock.release() # new threads can be spawned while we were waiting for the other # threads to complete def main_thread(): """Return the main thread object. In normal conditions, the main thread is the thread from which the Python interpreter was started. """ return _main_thread # get thread-local implementation, either from the thread # module, or from the python fallback try: from _thread import _local as local except ImportError: from _threading_local import local def _after_fork(): """ Cleanup threading module state that should not exist after a fork. """ # Reset _active_limbo_lock, in case we forked while the lock was held # by another (non-forked) thread. http://bugs.python.org/issue874900 global _active_limbo_lock, _main_thread global _shutdown_locks_lock, _shutdown_locks _active_limbo_lock = RLock() # fork() only copied the current thread; clear references to others. new_active = {} try: current = _active[get_ident()] except KeyError: # fork() was called in a thread which was not spawned # by threading.Thread. For example, a thread spawned # by thread.start_new_thread(). current = _MainThread() _main_thread = current # reset _shutdown() locks: threads re-register their _tstate_lock below _shutdown_locks_lock = _allocate_lock() _shutdown_locks = set() with _active_limbo_lock: # Dangling thread instances must still have their locks reset, # because someone may join() them. threads = set(_enumerate()) threads.update(_dangling) for thread in threads: # Any lock/condition variable may be currently locked or in an # invalid state, so we reinitialize them. if thread is current: # There is only one active thread. We reset the ident to # its new value since it can have changed. thread._reset_internal_locks(True) ident = get_ident() thread._ident = ident new_active[ident] = thread else: # All the others are already stopped. thread._reset_internal_locks(False) thread._stop() _limbo.clear() _active.clear() _active.update(new_active) assert len(_active) == 1 if hasattr(_os, "register_at_fork"): _os.register_at_fork(after_in_child=_after_fork)