keywords with a pattern of __* are class private names.
http://docs.python.org/reference/lexical_analysis.html#reserved-classes-of-identifiers
Quoting:
Names in this category, when used within the context of a class definition, are re-written to use a mangled form to help avoid name clashes between “private” attributes of base and derived classes
Private name mangling (emphasis added):
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
__spamoccurring 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.
http://docs.python.org/reference/expressions.html#atom-identifiers
This means that behind the scenes, B.__a() is transformed to something like B._B__a()