Leading double underscore names are private (meaning not available to derived classes)
This is not foolproof. It is implemented by mangling the name. Python Documentation says:
Any identifier of the form __spam (at least two leading underscores,
at most one trailing underscore) is textually replaced with
_classname__spam, where classname is the current class name with leading underscore(s) stripped. This mangling is done without regard
to the syntactic position of the identifier, so it can be used to
define class-private instance and class variables, methods, variables
stored in globals, and even variables stored in instances. private to
this class on instances of other classes.
Thus __get is actually mangled to _A__get in class A. When class B attempts to reference __get, it gets mangled to _B__get which doesn’t match.
In other words __plugh defined in class Xyzzy means “unless you are running as class Xyzzy, thou shalt not touch the __plugh.”