gevent.coros

class Semaphore(value=1) → Semaphore

Bases: object

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 with statements.

This Semaphore’s __exit__ method does not call the trace function on CPython, but does under PyPy.

See also

BoundedSemaphore for a safer version that prevents some classes of bugs.

acquire(blocking=True, timeout=None) → bool

Acquire the semaphore.

Caution

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).

Parameters:
  • blocking (bool) – If True (the default), this function will block until the semaphore is acquired.
  • timeout (float) – If given, specifies the maximum amount of seconds this method will block.
Returns:

A boolean indicating whether the semaphore was acquired. If blocking is True and timeout is None (the default), then (so long as this semaphore was initialized with a size greater than 0) this will always return True. If a timeout was given, and it expired before the semaphore was acquired, False will be returned. (Note that this can still raise a Timeout exception, if some other caller had already started a timer.)

locked()

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.

This method is normally called automatically by acquire() and wait(); most code will not need to use it.

release()

Release the semaphore, notifying any waiters if needed.

Remove the callback set by rawlink().

This method is normally called automatically by acquire() and wait(); most code will not need to use it.

wait(timeout=None) → int

Wait until it is possible to acquire this semaphore, or until the optional timeout elapses.

Caution

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.
class DummySemaphore(value=None) → DummySemaphore

Bases: object

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 Semaphore.

Changed in version 1.1rc3: Accept and ignore a value argument for compatibility with Semaphore.

acquire(blocking=True, timeout=None)

A DummySemaphore can always be acquired immediately so this always returns True and ignores its arguments.

locked()

A DummySemaphore is never locked so this always returns False.

release()

Releasing a dummy semaphore does nothing.

wait(timeout=None)

Waiting for a DummySemaphore returns immediately.

class BoundedSemaphore(value=1) → BoundedSemaphore

Bases: gevent._semaphore.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.

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