>>> import threading
>>> class foo (object):
...     def __bool__ (self):
...         return False
...     def __call__ (self):
...         print ("Running")
...
>>> threading.Thread (target = foo ()).start ()

The expected result of these commands would be for the thread to print
"Running". However, in actual fact it prints nothing at all. This is
because threading.Thread.run only runs the target if it is True as a
boolean. This is presumably to make the thread do nothing at all if
the target is None. In this case, I have a legitimate target that is
False as a boolean.

I propose to remove the test altogether. The effect of this is that
failure to set the target of the thread, or setting a non-callable
target, will cause the thread to raise a TypeError as soon as it is
started. Forgetting to set the target is in almost every case a bug,
and bugs should never be silent.

PR to follow.

New changeset 01c4fddc4b2ac707f226e0bd92679588d2252cc4 by BarneyStratford in branch 'master':
bpo-41149: Fix a bug in threading that causes fals-y threads callables to fail to start. (GH-21201)
https://github.com/python/cpython/commit/01c4fddc4b2ac707f226e0bd92679588d2252cc4