The statement
if A:
will call A.__bool__()
(see Special method names documentation), which was called __nonzero__
in Python 2, and use the return value of that function. Here’s the summary:
object.__bool__(self)
Called to implement truth value testing and the built-in operation
bool()
; should returnFalse
orTrue
. When this method is not defined,__len__()
is called, if it is defined, and the object is considered true if its result is nonzero. If a class defines neither__len__()
nor__bool__()
, all its instances are considered true.
On the other hand,
if A is not None:
compares only the reference A
with None
to see whether it is the same or not.