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Communicating Between Threads in Python
Communicating between threads in Python involves exchanging data or signals between different threads of execution. There are several mechanisms available for inter-thread communication in Python, including shared data structures, synchronization primitives, and message passing. Let's explore these mechanisms in detail.
Shared Data Structures
Shared data structures allow threads to read and modify shared data. However, care must be taken to ensure thread safety and avoid race conditions. Some commonly used shared data structures in Python are as mentioned below.
Locks (threading.Lock) − A lock provides mutual exclusion, allowing only one thread to acquire the lock at a time. It ensures that critical sections of code are executed atomically, preventing concurrent access to shared resources.
Semaphores (threading.Semaphore) − Semaphores maintain a count and allow multiple threads to acquire or release the semaphore. It can be used to control access to a limited number of resources.
Queues (queue.Queue) − Queues provide thread-safe data structures for exchanging data between threads. The `Queue` class in the `queue` module provides methods like `put()` and `get()` to enqueue and dequeue elements, respectively.
Shared variables with locks − Threads can share data using shared variables protected by locks. By acquiring the lock before accessing or modifying the shared variable, we can ensure that only one thread operates on it at a time.
Synchronization Primitives
Synchronization primitives help co-ordinate the execution of multiple threads to ensure proper ordering or synchronization. They are used to control access to shared resources and establish a consistent state among threads. Python provides several synchronization primitives as below.
Event (threading.Event) − Events are used to notify one or more threads about a particular condition or state change. Threads can wait for an event to be set or cleared using the `wait()` method and set or clear the event using the `set()` and `clear()` methods, respectively.
Condition (threading.Condition) − Conditions are advanced synchronization primitives that allow threads to wait until a certain condition is met. They can be used in combination with locks to enable threads to wait for a specific condition and signal when the condition is satisfied.
Barrier (threading.Barrier) − Barriers allow a group of threads to synchronize at a particular point, where they wait until all threads have reached the barrier before proceeding.
Semaphore (threading.Semaphore) − As mentioned earlier, semaphores can be used to control access to a limited number of resources.
Message Passing
Message passing involves sending messages or signals between threads for communication. This can be achieved using various techniques as mentioned below.
Thread-safe queues: Queues can be used to pass messages between threads. One thread enqueues messages using the `put()` method, and another thread dequeues messages using the `get()` method.
Event-driven programming − Threads can communicate through events and callbacks. One thread can trigger events or invoke callbacks that are handled by another thread.
Explicit communication channels − Custom communication channels can be established using shared data structures or synchronization primitives to pass messages between threads explicitly.
It's important to select the appropriate mechanism for inter-thread communication based on the specific requirements of our application. Consider factors such as thread safety, synchronization, data sharing, and the nature of the communication needed between threads.
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