if A vs if A is not None:

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 return False or True. 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.