None or 0

[Terry Reedy]

To summarize Tim's thorough answer:
the language reference guarantees (specifies) the short-circuit
behavior
that you discovered.  It also guarantees that
> a = a or b
is *not* equivalent to
> if a is None:   a = b
unless *you* can guarantee that 'a' will never have a null value other
than None.

I add: as far as I know, both have been true since the beginning and
will remain true thru the 2.x series.  I would be extremely surprised
if anything changed thereafter.

Terry J. Reedy