This module implements synchronization primitives to be used with cooperative tasks, in an analogous and API compatible way as threading module’s primitives which are used with threads.
A semaphore manages an internal counter which is decremented by each acquire() call and incremented by each release() call. The counter can never go below zero; when acquire() finds that it is zero, it blocks, waiting until some other task calls release().
The optional argument gives the initial value for the internal counter; it defaults to 1. If the value given is less than 0, ValueError is raised.
Acquire a semaphore.
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 task 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 tasks are awakened should not be relied on. Returns true (or blocks indefinitely).
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.
Release a semaphore, incrementing the internal counter by one. When it was zero on entry and another task is waiting for it to become larger than zero again, wake up that task.
Class implementing bounded semaphore objects. 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.
This class implements reentrant lock objects. A reentrant lock must be released by the task that acquired it. Once a task has acquired a reentrant lock, the same task may acquire it again without blocking; the task must release it once for each time it has acquired it.
Acquire a lock, blocking or non-blocking.
When invoked without arguments: if this task already owns the lock, increment the recursion level by one, and return immediately. Otherwise, if another task owns the lock, block until the lock is unlocked. Once the lock is unlocked (not owned by any task), then grab ownership, set the recursion level to one, and return. If more than one task 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.
Release a lock, decrementing the recursion level. If after the decrement it is zero, reset the lock to unlocked (not owned by any task), and if any other tasks 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 task.
Only call this method when the calling task owns the lock. A RuntimeError is raised if this method is called when the lock is unlocked.
There is no return value.
This class implements condition variable objects. A condition variable allows one or more tasks to wait until they are notified by another task.
If the lock argument is given and not None, it must be a Semaphore or RLock object, and it is used as the underlying lock. Otherwise, a new RLock object is created and used as the underlying lock.
Acquire the underlying lock. This method calls the corresponding method on the underlying lock; the return value is whatever that method returns.
Release the underlying lock. This method calls the corresponding method on the underlying lock; there is no return value.
Wait until notified or until a timeout occurs. If the calling task 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 task, 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.
The return value is True unless a given timeout expired, in which case it is False.
By default, wake up one task waiting on this condition, if any. If the calling task has not acquired the lock when this method is called, a RuntimeError is raised.
This method wakes up at most n of the tasks waiting for the condition variable; it is a no-op if no tasks are waiting.
The current implementation wakes up exactly n tasks, if at least n tasks are waiting. However, it’s not safe to rely on this behavior. A future, optimized implementation may occasionally wake up more than n tasks.
Note: an awakened task does not actually return from its wait() call until it can reacquire the lock. Since notify() does not release the lock, its caller should.