Python double underscore mangling

I am a bit confused by this behavior (using python 3.2):

class Bar:
    pass

bar = Bar()
bar.__cache = None
print(vars(bar))        # {'__cache': None}

class Foo:
    def __init__(self):
        self.__cache = None

foo = Foo()
print(vars(foo))        # {'_Foo__cache': None}

I've read up a bit on how double-underscores cause attribute names to be "mangled", but I would have expected the same name-mangling in both cases above.

What is the meaning of a single- and a double-underscore before an object name?

Any ideas what's going on here?

Name mangling occurs during the evaluation of a class statement. In the case of Bar, the __cache attribute is not defined as part of the class, but rather added to a specific object after the fact.

(Actually, that may not be entirely correct. Name mangling may occur during the evaluation of the __new__ method; I do not know. But regardless, your __cache is added explicitly to a single object, not added by the class code.)