Join and Status

Use is_alive and join so the Python main thread can see whether a child is running and can wait at one line until that child finishes.

Example

import threading
import time

def squares(values):
    time.sleep(0.05)
    print([p * p for p in values])

worker = threading.Thread(target=squares, args=([1, 2, 3],), name="squares")
print("before", worker.is_alive(), threading.active_count())
worker.start()
print("started", worker.is_alive())
worker.join()
print("joined", worker.is_alive(), threading.active_count())

Before start(), is_alive() is false and active_count() is 1. After start(), the child counts as active while the short sleep keeps it alive. join() pauses MainThread at that line until squares finishes, so the joined line runs only after [1, 4, 9] prints. join() orders the statements. A normal child is not discarded when main reaches its last line; the process still waits for that child. Use join() when later main-thread work must happen after the child.

Key takeaways

• is_alive() is false before start() and after the child finishes

• is_alive() is true while that child is running

• active_count() includes MainThread

• join() pauses the caller until that child finishes

• Later statements in main run after join() returns

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