**When and Why Should You Use `join()` in Python Threading?**

Join() in Threading: Understanding Its Usage

Python's threading module provides the join() method to synchronize the execution of multiple threads. The primary purpose of join() is to ensure that a thread completes its execution before the main thread terminates.

Usage in Daemon Threads

The main thread generally waits for all non-daemon threads to complete before exiting. However, daemon threads run in the background and terminate automatically when the main thread finishes. Therefore, calling join() on a daemon thread is unnecessary.

Usage in Non-Daemon Threads

Interestingly, join() can also be used for non-daemon threads, even though it's not strictly required. Here's an example where join() is applied to both daemon and non-daemon threads:

<code class="python">import threading
import logging

# Configure logging
logging.basicConfig(level=logging.DEBUG,
                    format='(%(threadName)-10s) %(message)s',
                    )

# Define a daemon thread
def daemon():
    logging.debug('Starting')
    time.sleep(2)
    logging.debug('Exiting')

# Create and start a daemon thread
d = threading.Thread(name='daemon', target=daemon)
d.setDaemon(True)
d.start()

# Define a non-daemon thread
def non_daemon():
    logging.debug('Starting')
    logging.debug('Exiting')

# Create and start a non-daemon thread
t = threading.Thread(name='non-daemon', target=non_daemon)
t.start()

# Join both threads
d.join()
t.join()</code>

Mechanism of Join()

The join() method waits for the target thread to complete its execution. If the target thread is non-daemon, the main thread will wait indefinitely for it to finish. This ensures that the main thread doesn't terminate until all non-daemon threads have completed.

Visual Representation

The following ASCII-art demonstrates the behavior of join():

+---+---+------------------***********+###
|   |   |                             |
|   +...........join()            |        child-thread(short)
+......................join()......        child-thread(long)

'-' Main thread execution
'.' Child thread execution
'#' Parent thread execution after join()
'*' Main thread sleeping in join()
',' Daemonized thread

Conclusion

While join() is primarily used for daemon threads, it can also be applied to non-daemon threads to ensure their completion before the main thread exits. Understanding the mechanism of join() is crucial for effective thread management in Python.

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