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I've just run into a situation where pseudo -private class member names aren't getting mangled when using setattr or exec . In [1]: class T: ...: def __init__(self, **kwargs): ...: self.__x = 1 ...: for k, v in kwargs.items(): ...: setattr(self, "__%s" % k, v) ...: In [2]: T(y=2).__dict__ Out[2]: {'_T__x': 1, '__y': 2} I've tried exec("self.__%s = %s" % (k, v)) as well with the same result: In [1]: class T: ...: def __init__(self, **kwargs): ...: self.__x = 1 ...: for k, v in kwargs.items(): ...: exec("self.__%s = %s" % (k, v)) ...: In [2]: T(z=3).__dict__ Out[2]: {'_T__x': 1, '__z': 3} Doing self.__dict__["_%s__%s" % (self.__class__.__name__, k)] = v would work, but __dict__ is a readonly attribute. Is there another way that I can dynamically create these psuedo -private class members (without hard-coding in the name mangling)? A better way to phrase my question: What does python do “under the hood” when it encounters a double underscore ( self.__x ) attribute being set? Is there a magic function that is used to do the mangling?

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