t.join

Describing join(timeout=None)

• The join method waits until:

  • The thread terminates
  • Or a timeout occurs
• The timeout represents the seconds until timeout
• In t.join(), the main thread will be blocked until thread t finishes executing
• Then, the main thread waits until thread t has finished
• Once t finishes, then the main thread can resume execution
• Specifically, the main thread starts executes code after join
• Essentially, the main thread is paused until thread t finishes

Caveats of join

• A thread cannot join itself
• A thread cannot join before it has started
• Note, the scheduling of threads is done by the OS
• Implying, the order in which threads are run is determined by the operating system
• Meaning, multiple runs will produce different orderings
• Each thread's order of execution will vary from run to run

Setup Code for join

# sleepy.py
>>> import logging
>>> from threading import Thread
>>> import time

>>> def sleepy():
...     print('thread started')
...     time.sleep(10)
...     print('thread done')

Example without join

>>> t = Thread(target=sleepy)
>>> t.start()
thread started
>>> print('main done')
main done
thread done

Example with join

>>> t = Thread(target=sleepy)
>>> t.start()
thread started
>>> t.join()  # haults any further execution
thread done   # until thread finishes
>>> print('main done')  # now we can print
main done

References

• Python Essential References
• Documentation for Threading API
• Examples of Threading in Python
• Example of Joining Threads
• Description of Blocking the Main Thread
• Illustrating Main Thread Sleeping