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Get Started Now!Threads share the memory allocated to a process. As a result, threads in the same process can communicate with each other. To facilitate inter-thread communication, the threading module provides Event object and Condition object.
The Event Object
An Event object manages the state of an internal flag. The flag is initially false and becomes true with the set() method and reset to false with the clear() method. The wait() method blocks until the flag is true.
Methods of Event object −
is_set() method
Return True if and only if the internal flag is true.
set() method
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.
clear() method
Reset the internal flag to false. Subsequently, threads calling wait() will block until set() is called to set the internal flag to true again.
wait(timeout=None) method
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.
Example
The following code attempts to simulate the traffic flow being controlled by the state of traffic signal either GREEN or RED.
There are two threads in the program, targeting two different functions. The signal_state() function periodically sets and resets the event indicating change of signal from GREEN to RED.
The traffic_flow() function waits for the event to be set, and runs a loop till it remains set.
Output
Waiting for GREEN Signal Traffic Police Giving GREEN Signal GREEN Signal ... Traffic can move Vehicle No: 1 Crossing the Signal Vehicle No: 2 Crossing the Signal Vehicle No: 3 Crossing the Signal Vehicle No: 4 Crossing the Signal Vehicle No: 5 Crossing the Signal Signal is RED RED Signal ... Traffic has to wait Waiting for GREEN Signal Traffic Police Giving GREEN Signal GREEN Signal ... Traffic can move Vehicle No: 6 Crossing the Signal Vehicle No: 7 Crossing the Signal Vehicle No: 8 Crossing the Signal Vehicle No: 9 Crossing the Signal Vehicle No: 10 Crossing the Signal
The Condition Object
Condition class in threading module class implements condition variable objects. Condition object forces one or more threads to wait until notified by another thread. Condition is associated with a Reentrant Lock. A condition object has acquire() and release() methods that call the corresponding methods of the associated lock.
Following are the methods of the Condition object −
acquire(*args)
Acquire the underlying lock. This method calls the corresponding method on the underlying lock; the return value is whatever that method returns.
release()
Release the underlying lock. This method calls the corresponding method on the underlying lock; there is no return value.
wait(timeout=None)
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.
wait_for(predicate, timeout=None)
This utility method may call wait() repeatedly until the predicate is satisfied, or until a timeout occurs. The return value is the last return value of the predicate and will evaluate to False if the method timed out.
notify(n=1)
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.
notify_all()
Wake up all threads waiting on this condition. This method acts like notify(), but wakes up all waiting threads instead of one. If the calling thread has not acquired the lock when this method is called, a RuntimeError is raised.
Example
In the following code, the thread t2 runs taskB() function and t1 runs taskA() function. The t1 thread acquires the condition and notifies it. By that time the t2 thread is in waiting state. After the condition is released, the waiting thread proceeds to consume the random number generated by the notifying function.
When you execute this code, it will produce the following output −
waiting for update Generated random number: 4 Notification issued Obtained random number 4 waiting for update Generated random number: 6 Notification issued Obtained random number 6 waiting for update Generated random number: 10 Notification issued Obtained random number 10 waiting for update
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