An identifier occurring as an atom is a name. See Section 4.1 for
documentation of naming and binding.
When the name is bound to an object, evaluation of the atom yields
that object. When a name is not bound, an attempt to evaluate it
raises a NameError exception.
Private name mangling:when an identifier that textually occurs in a class definition begins
with two or more underscore characters and does not end in two or more
underscores, it is considered a private name of that class.
Private names are transformed to a longer form before code is
generated for them. The transformation inserts the class name in
front of the name, with leading underscores removed, and a single
underscore inserted in front of the class name. For example, the
identifier __spam occurring in a class named Ham will be
transformed to _Ham__spam. This transformation is independent
of the syntactical context in which the identifier is used. If the
transformed name is extremely long (longer than 255 characters),
implementation defined truncation may happen. If the class name
consists only of underscores, no transformation is done.
