To make a Python thread wait for a signal (like a boolean switch) from another thread without polling, you can use the threading.Event class. This is a synchronization primitive that allows one or more threads to wait until an event is set by another thread.

How It Works:

• threading.Event: This object provides a simple way for threads to communicate. One thread can set the event, and other threads can wait for it to be set.
• No Polling: Threads waiting for the event will block until the event is set, avoiding the inefficiency of polling.

Example Code:

Here’s how you can implement it:

import threading
import time

# Create an Event object
event = threading.Event()

def worker_thread():
    print("Worker thread: Waiting for the signal...")
    # Wait for the event to be set
    event.wait()
    print("Worker thread: Received the signal! Continuing...")

def main_thread():
    print("Main thread: Starting worker thread...")
    worker = threading.Thread(target=worker_thread)
    worker.start()

    # Simulate some work in the main thread
    time.sleep(2)

    print("Main thread: Sending the signal!")
    # Set the event to signal the worker thread
    event.set()

    worker.join()
    print("Main thread: Worker thread has finished.")

# Run the main thread
main_thread()

Explanation:

1. event = threading.Event(): Creates an Event object.
2. event.wait(): The worker thread calls this method and blocks until the event is set.
3. event.set(): The main thread sets the event, which unblocks the worker thread.

Key Points:

• Efficiency: No CPU cycles are wasted on polling.
• Thread Safety: The Event object is thread-safe and handles synchronization internally.
• Reusability: You can reset the event with event.clear() if needed.