How to receive thread callbacks in Python?

Multithreading is a powerful concept in programming, allowing developers to execute multiple tasks concurrently and improve overall program performance. In Python, the threading module provides a convenient way to implement multithreading. When working with threads, it's often necessary to receive callbacks to handle events or synchronize the execution of different threads. In this tutorial, we'll explore various techniques for receiving thread callbacks in Python.

Threads in Python

Before delving into thread callbacks, let's briefly review the basics of threading in Python. The threading module offers a high-level interface for creating and managing threads. Threads are lightweight processes that share the same memory space but run independently.

Example

Here's a simple example of creating and running a thread in Python −

import threading

def my_function():
   print("This is a thread.")

# Create a thread
my_thread = threading.Thread(target=my_function)

# Start the thread
my_thread.start()

# Wait for the thread to finish
my_thread.join()

Output

This is a thread.

The Need for Thread Callbacks

When working with threads, it's common to encounter scenarios where one thread needs to notify another about a particular event or completion of a task. This is where thread callbacks become essential. A thread callback is a mechanism through which one thread can execute a specified function in response to an event in another thread.

Let's now discuss the techniques for receiving thread callbacks in Python.

Using Events for Thread Callbacks

One approach to implementing thread callbacks is by using the Event class from the threading module. An event is a simple synchronization primitive that allows one thread to signal another that a certain event has occurred.

Example

Let's consider an example where a worker thread performs a task and signals the main thread using an event −

import threading
import time

def worker(callback_event):
   print("Worker is performing a task.")
   time.sleep(2)
   callback_event.set()

def callback_function():
   print("Callback received: Task is complete!")

if __name__ == "__main__":
   # Create an event to signal the callback
   callback_event = threading.Event()

   # Create a thread with the worker function and pass the callback event
   worker_thread = threading.Thread(target=worker, args=(callback_event,))

   # Start the worker thread
   worker_thread.start()

   # Wait for the callback event to be set
   callback_event.wait()

   # Perform the callback action
   callback_function()

   # Wait for the worker thread to finish
   worker_thread.join()

Output

In this example, the worker thread performs a task and sets the callback_event to signal the main thread. The main thread waits for the event to be set and then executes the callback_function.

Worker is performing a task.
Callback received: Task is complete!

Using Queues for Thread Callbacks

Another approach for receiving thread callbacks is by using queues. The Queue class from the queue module provides a thread-safe way to exchange data between threads.

Example

Consider the following example where the worker thread puts a message into a queue, and the main thread retrieves and processes the message −

import threading
import queue
import time

def worker(callback_queue):
   print("Worker is performing a task.")
   time.sleep(2)
   callback_queue.put("Task is complete!")

def callback_function(message):
   print(f"Callback received: {message}")

if __name__ == "__main__":
   # Create a queue for callbacks
   callback_queue = queue.Queue()

   # Create a thread with the worker function and pass the callback queue
   worker_thread = threading.Thread(target=worker, args=(callback_queue,))

   # Start the worker thread
   worker_thread.start()

   # Wait for the worker thread to finish
   worker_thread.join()

   # Retrieve and process the callback message
   callback_message = callback_queue.get()
   callback_function(callback_message)

Output

In this example, the worker thread puts a message into the callback_queue, and the main thread retrieves and processes the message using the callback_function.

Worker is performing a task.
Callback received: Task is complete!

Choosing Between Events and Queues

The choice between using events or queues for thread callbacks depends on the complexity of the data you need to exchange and the synchronization requirements of your program.

• Events − Events are suitable for simple signaling between threads. If you only need to notify another thread that a specific event has occurred, events provide a lightweight solution.

• Queues − Queues are more versatile and can handle more complex communication between threads. If you need to exchange data or messages between threads, queues provide a thread-safe and efficient mechanism.

Handling Multiple Callbacks

In real-world applications, you might encounter scenarios where multiple threads need to register and receive callbacks. This can be achieved by maintaining a list of callbacks and iterating over them when needed.

Example

Let's modify the previous example to handle multiple callbacks −

import threading
import queue
import time

def worker(callbacks):
   print("Worker is performing a task.")
   time.sleep(2)
   for callback in callbacks:
      callback.put("Task is complete!")

def callback_function(message):
   print(f"Callback received: {message}")

if __name__ == "__main__":
   # Create a list of queues for callbacks
   callback_queues = [queue.Queue() for _ in range(3)]

   # Create a thread with the worker function and pass the callback queues
   worker_thread = threading.Thread(target=worker, args=(callback_queues,))

   # Start the worker thread
   worker_thread.start()

   # Wait for the worker thread to finish
   worker_thread.join()

   # Retrieve and process the callback messages
   for callback_queue in callback_queues:
      callback_message = callback_queue.get()
      callback_function(callback_message)

Output

In this modified example, we create a list of queues (callback_queues) and pass it to the worker thread. The worker thread iterates over the list, putting messages into each queue. The main thread then retrieves and processes the messages from each queue.

Worker is performing a task.
Callback received: Task is complete!
Callback received: Task is complete!
Callback received: Task is complete!

Conclusion

Implementing thread callbacks in Python involves selecting the appropriate mechanism based on the requirements of your application. Whether you choose events or queues, understanding the synchronization primitives provided by the threading module is crucial for writing robust multithreaded code.