Semaphore(value=1) → Semaphore¶
A semaphore manages 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.
The semaphore is a context manager and can be used in
__exit__ method does not call the trace function
on CPython, but does under PyPy.
BoundedSemaphore for a safer version that prevents
some classes of bugs.
acquire(blocking=True, timeout=None) → bool¶
Acquire the semaphore.
If this semaphore was initialized with a size of 0, this method will block forever (unless a timeout is given or blocking is set to false).
A boolean indicating whether the semaphore was acquired.
Return a boolean indicating whether the semaphore can be acquired. Most useful with binary semaphores.
Register a callback to call when a counter is more than zero.
callback will be called in the
Hub, so it must not use blocking gevent API.
callback will be passed one argument: this instance.
Release the semaphore, notifying any waiters if needed.
Remove the callback set by
wait(timeout=None) → int¶
Wait until it is possible to acquire this semaphore, or until the optional timeout elapses.
If this semaphore was initialized with a size of 0, this method will block forever if no timeout is given.
|Parameters:||timeout (float) – If given, specifies the maximum amount of seconds this method will block.|
|Returns:||A number indicating how many times the semaphore can be acquired before blocking.|
DummySemaphore(value=None) → DummySemaphore¶
A Semaphore initialized with “infinite” initial value. None of its methods ever block.
This can be used to parameterize on whether or not to actually
guard access to a potentially limited resource. If the resource is
actually limited, such as a fixed-size thread pool, use a real
Semaphore, but if the resource is unbounded, use an
instance of this class. In that way none of the supporting code
needs to change.
Similarly, it can be used to parameterize on whether or not to
enforce mutual exclusion to some underlying object. If the
underlying object is known to be thread-safe itself mutual
exclusion is not needed and a
DummySemaphore can be used, but
if that’s not true, use a real
Changed in version 1.1rc3: Accept and ignore a value argument for compatibility with Semaphore.
A DummySemaphore can always be acquired immediately so this always returns True and ignores its arguments.
A DummySemaphore is never locked so this always returns False.
Releasing a dummy semaphore does nothing.
Waiting for a DummySemaphore returns immediately.
BoundedSemaphore(value=1) → BoundedSemaphore¶
A bounded semaphore checks to make sure its current value doesn’t
exceed its initial value. If it does,
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.