Thread.join

Wait until the thread terminates.

This blocks the calling thread until the thread whose join() method is
called terminates -- either normally or through an unhandled exception
or until the optional timeout occurs.

When the timeout argument is present and not None, it should be a
floating-point number specifying a timeout for the operation in seconds
(or fractions thereof). As join() always returns None, you must call
is_alive() after join() to decide whether a timeout happened -- if the
thread is still alive, the join() call timed out.

When the timeout argument is not present or None, the operation will
block until the thread terminates.

A thread can be join()ed many times.

join() raises a RuntimeError if an attempt is made to join the current
thread as that would cause a deadlock. It is also an error to join() a
thread before it has been started and attempts to do so raises the same
exception.