507 lines
16 KiB
Python
507 lines
16 KiB
Python
"""Synchronization primitives."""
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__all__ = ('Lock', 'Event', 'Condition', 'Semaphore', 'BoundedSemaphore')
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import collections
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import warnings
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from . import events
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from . import futures
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from .coroutines import coroutine
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class _ContextManager:
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"""Context manager.
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This enables the following idiom for acquiring and releasing a
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lock around a block:
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with (yield from lock):
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<block>
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while failing loudly when accidentally using:
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with lock:
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<block>
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Deprecated, use 'async with' statement:
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async with lock:
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<block>
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"""
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def __init__(self, lock):
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self._lock = lock
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def __enter__(self):
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# We have no use for the "as ..." clause in the with
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# statement for locks.
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return None
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def __exit__(self, *args):
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try:
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self._lock.release()
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finally:
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self._lock = None # Crudely prevent reuse.
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class _ContextManagerMixin:
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def __enter__(self):
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raise RuntimeError(
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'"yield from" should be used as context manager expression')
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def __exit__(self, *args):
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# This must exist because __enter__ exists, even though that
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# always raises; that's how the with-statement works.
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pass
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@coroutine
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def __iter__(self):
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# This is not a coroutine. It is meant to enable the idiom:
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#
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# with (yield from lock):
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# <block>
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#
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# as an alternative to:
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#
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# yield from lock.acquire()
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# try:
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# <block>
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# finally:
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# lock.release()
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# Deprecated, use 'async with' statement:
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# async with lock:
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# <block>
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warnings.warn("'with (yield from lock)' is deprecated "
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"use 'async with lock' instead",
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DeprecationWarning, stacklevel=2)
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yield from self.acquire()
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return _ContextManager(self)
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async def __acquire_ctx(self):
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await self.acquire()
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return _ContextManager(self)
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def __await__(self):
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warnings.warn("'with await lock' is deprecated "
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"use 'async with lock' instead",
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DeprecationWarning, stacklevel=2)
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# To make "with await lock" work.
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return self.__acquire_ctx().__await__()
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async def __aenter__(self):
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await self.acquire()
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# We have no use for the "as ..." clause in the with
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# statement for locks.
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return None
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async def __aexit__(self, exc_type, exc, tb):
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self.release()
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class Lock(_ContextManagerMixin):
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"""Primitive lock objects.
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A primitive lock is a synchronization primitive that is not owned
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by a particular coroutine when locked. A primitive lock is in one
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of two states, 'locked' or 'unlocked'.
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It is created in the unlocked state. It has two basic methods,
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acquire() and release(). When the state is unlocked, acquire()
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changes the state to locked and returns immediately. When the
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state is locked, acquire() blocks until a call to release() in
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another coroutine changes it to unlocked, then the acquire() call
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resets it to locked and returns. The release() method should only
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be called in the locked state; it changes the state to unlocked
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and returns immediately. If an attempt is made to release an
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unlocked lock, a RuntimeError will be raised.
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When more than one coroutine is blocked in acquire() waiting for
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the state to turn to unlocked, only one coroutine proceeds when a
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release() call resets the state to unlocked; first coroutine which
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is blocked in acquire() is being processed.
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acquire() is a coroutine and should be called with 'await'.
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Locks also support the asynchronous context management protocol.
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'async with lock' statement should be used.
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Usage:
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lock = Lock()
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...
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await lock.acquire()
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try:
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...
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finally:
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lock.release()
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Context manager usage:
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lock = Lock()
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...
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async with lock:
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...
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Lock objects can be tested for locking state:
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if not lock.locked():
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await lock.acquire()
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else:
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# lock is acquired
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...
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"""
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def __init__(self, *, loop=None):
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self._waiters = collections.deque()
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self._locked = False
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if loop is not None:
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self._loop = loop
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else:
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self._loop = events.get_event_loop()
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def __repr__(self):
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res = super().__repr__()
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extra = 'locked' if self._locked else 'unlocked'
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if self._waiters:
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extra = f'{extra}, waiters:{len(self._waiters)}'
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return f'<{res[1:-1]} [{extra}]>'
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def locked(self):
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"""Return True if lock is acquired."""
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return self._locked
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async def acquire(self):
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"""Acquire a lock.
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This method blocks until the lock is unlocked, then sets it to
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locked and returns True.
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"""
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if not self._locked and all(w.cancelled() for w in self._waiters):
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self._locked = True
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return True
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fut = self._loop.create_future()
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self._waiters.append(fut)
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# Finally block should be called before the CancelledError
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# handling as we don't want CancelledError to call
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# _wake_up_first() and attempt to wake up itself.
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try:
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try:
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await fut
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finally:
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self._waiters.remove(fut)
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except futures.CancelledError:
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if not self._locked:
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self._wake_up_first()
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raise
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self._locked = True
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return True
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def release(self):
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"""Release a lock.
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When the lock is locked, reset it to unlocked, and return.
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If any other coroutines are blocked waiting for the lock to become
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unlocked, allow exactly one of them to proceed.
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When invoked on an unlocked lock, a RuntimeError is raised.
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There is no return value.
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"""
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if self._locked:
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self._locked = False
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self._wake_up_first()
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else:
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raise RuntimeError('Lock is not acquired.')
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def _wake_up_first(self):
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"""Wake up the first waiter if it isn't done."""
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try:
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fut = next(iter(self._waiters))
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except StopIteration:
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return
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# .done() necessarily means that a waiter will wake up later on and
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# either take the lock, or, if it was cancelled and lock wasn't
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# taken already, will hit this again and wake up a new waiter.
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if not fut.done():
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fut.set_result(True)
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class Event:
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"""Asynchronous equivalent to threading.Event.
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Class implementing event objects. An event manages a flag that can be set
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to true with the set() method and reset to false with the clear() method.
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The wait() method blocks until the flag is true. The flag is initially
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false.
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"""
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def __init__(self, *, loop=None):
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self._waiters = collections.deque()
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self._value = False
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if loop is not None:
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self._loop = loop
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else:
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self._loop = events.get_event_loop()
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def __repr__(self):
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res = super().__repr__()
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extra = 'set' if self._value else 'unset'
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if self._waiters:
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extra = f'{extra}, waiters:{len(self._waiters)}'
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return f'<{res[1:-1]} [{extra}]>'
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def is_set(self):
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"""Return True if and only if the internal flag is true."""
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return self._value
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def set(self):
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"""Set the internal flag to true. All coroutines waiting for it to
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become true are awakened. Coroutine that call wait() once the flag is
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true will not block at all.
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"""
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if not self._value:
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self._value = True
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for fut in self._waiters:
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if not fut.done():
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fut.set_result(True)
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def clear(self):
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"""Reset the internal flag to false. Subsequently, coroutines calling
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wait() will block until set() is called to set the internal flag
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to true again."""
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self._value = False
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async def wait(self):
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"""Block until the internal flag is true.
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If the internal flag is true on entry, return True
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immediately. Otherwise, block until another coroutine calls
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set() to set the flag to true, then return True.
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"""
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if self._value:
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return True
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fut = self._loop.create_future()
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self._waiters.append(fut)
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try:
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await fut
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return True
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finally:
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self._waiters.remove(fut)
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class Condition(_ContextManagerMixin):
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"""Asynchronous equivalent to threading.Condition.
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This class implements condition variable objects. A condition variable
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allows one or more coroutines to wait until they are notified by another
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coroutine.
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A new Lock object is created and used as the underlying lock.
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"""
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def __init__(self, lock=None, *, loop=None):
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if loop is not None:
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self._loop = loop
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else:
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self._loop = events.get_event_loop()
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if lock is None:
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lock = Lock(loop=self._loop)
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elif lock._loop is not self._loop:
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raise ValueError("loop argument must agree with lock")
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self._lock = lock
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# Export the lock's locked(), acquire() and release() methods.
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self.locked = lock.locked
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self.acquire = lock.acquire
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self.release = lock.release
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self._waiters = collections.deque()
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def __repr__(self):
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res = super().__repr__()
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extra = 'locked' if self.locked() else 'unlocked'
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if self._waiters:
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extra = f'{extra}, waiters:{len(self._waiters)}'
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return f'<{res[1:-1]} [{extra}]>'
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async def wait(self):
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"""Wait until notified.
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If the calling coroutine has not acquired the lock when this
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method is called, a RuntimeError is raised.
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This method releases the underlying lock, and then blocks
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until it is awakened by a notify() or notify_all() call for
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the same condition variable in another coroutine. Once
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awakened, it re-acquires the lock and returns True.
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"""
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if not self.locked():
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raise RuntimeError('cannot wait on un-acquired lock')
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self.release()
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try:
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fut = self._loop.create_future()
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self._waiters.append(fut)
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try:
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await fut
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return True
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finally:
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self._waiters.remove(fut)
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finally:
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# Must reacquire lock even if wait is cancelled
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cancelled = False
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while True:
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try:
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await self.acquire()
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break
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except futures.CancelledError:
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cancelled = True
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if cancelled:
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raise futures.CancelledError
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async def wait_for(self, predicate):
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"""Wait until a predicate becomes true.
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The predicate should be a callable which result will be
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interpreted as a boolean value. The final predicate value is
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the return value.
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"""
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result = predicate()
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while not result:
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await self.wait()
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result = predicate()
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return result
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def notify(self, n=1):
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"""By default, wake up one coroutine waiting on this condition, if any.
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If the calling coroutine has not acquired the lock when this method
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is called, a RuntimeError is raised.
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This method wakes up at most n of the coroutines waiting for the
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condition variable; it is a no-op if no coroutines are waiting.
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Note: an awakened coroutine does not actually return from its
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wait() call until it can reacquire the lock. Since notify() does
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not release the lock, its caller should.
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"""
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if not self.locked():
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raise RuntimeError('cannot notify on un-acquired lock')
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idx = 0
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for fut in self._waiters:
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if idx >= n:
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break
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if not fut.done():
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idx += 1
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fut.set_result(False)
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def notify_all(self):
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"""Wake up all threads waiting on this condition. This method acts
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like notify(), but wakes up all waiting threads instead of one. If the
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calling thread has not acquired the lock when this method is called,
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a RuntimeError is raised.
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"""
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self.notify(len(self._waiters))
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class Semaphore(_ContextManagerMixin):
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"""A Semaphore implementation.
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A semaphore manages an internal counter which is decremented by each
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acquire() call and incremented by each release() call. The counter
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can never go below zero; when acquire() finds that it is zero, it blocks,
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waiting until some other thread calls release().
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Semaphores also support the context management protocol.
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The optional argument gives the initial value for the internal
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counter; it defaults to 1. If the value given is less than 0,
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ValueError is raised.
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"""
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def __init__(self, value=1, *, loop=None):
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if value < 0:
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raise ValueError("Semaphore initial value must be >= 0")
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self._value = value
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self._waiters = collections.deque()
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if loop is not None:
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self._loop = loop
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else:
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self._loop = events.get_event_loop()
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def __repr__(self):
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res = super().__repr__()
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extra = 'locked' if self.locked() else f'unlocked, value:{self._value}'
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if self._waiters:
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extra = f'{extra}, waiters:{len(self._waiters)}'
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return f'<{res[1:-1]} [{extra}]>'
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def _wake_up_next(self):
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while self._waiters:
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waiter = self._waiters.popleft()
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if not waiter.done():
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waiter.set_result(None)
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return
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def locked(self):
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"""Returns True if semaphore can not be acquired immediately."""
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return self._value == 0
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async def acquire(self):
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"""Acquire a semaphore.
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If the internal counter is larger than zero on entry,
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decrement it by one and return True immediately. If it is
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zero on entry, block, waiting until some other coroutine has
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called release() to make it larger than 0, and then return
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True.
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"""
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while self._value <= 0:
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fut = self._loop.create_future()
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self._waiters.append(fut)
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try:
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await fut
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except:
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# See the similar code in Queue.get.
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fut.cancel()
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if self._value > 0 and not fut.cancelled():
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self._wake_up_next()
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raise
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self._value -= 1
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return True
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def release(self):
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"""Release a semaphore, incrementing the internal counter by one.
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When it was zero on entry and another coroutine is waiting for it to
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become larger than zero again, wake up that coroutine.
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"""
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self._value += 1
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self._wake_up_next()
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class BoundedSemaphore(Semaphore):
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"""A bounded semaphore implementation.
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This raises ValueError in release() if it would increase the value
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above the initial value.
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"""
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def __init__(self, value=1, *, loop=None):
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self._bound_value = value
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super().__init__(value, loop=loop)
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def release(self):
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if self._value >= self._bound_value:
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raise ValueError('BoundedSemaphore released too many times')
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super().release()
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