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isinstance() and multiple inheritance/ctypes

I'm running into some strangeness trying to work with the bitfield module 
from my ctypes-bitfield package (on PyPi).  I'm trying to use isinstance
(), and it's kinda sorta lying to me.

----- IPython session (Python 3.4 under Linux) -------
In [649]: bf.__mro__
Out[649]: (bitfield._TD, _ctypes.Union, _ctypes._CData, 
bitfield.Bitfield, builtins.object)

In [650]: isinstance(bf, bitfield.Bitfield)
Out[650]: False

In [651]: bf.__bases__
Out[651]: (_ctypes.Union, bitfield.Bitfield)

In [652]: bf.__bases__[1]
Out[652]: bitfield.Bitfield

In [653]: bf.__bases__[1] is bitfield.Bitfield
Out[653]: True

Is there an issue with isinstance and multiple inheritance?  Conversely 
is there an issue with isinstance and ctypes derived classes (Bitfield 
isn't, but Bitfield is a mixin that always works with Unions)  I know 
that ctypes classes can get really wonky on this stuff.

More generally, is there any good way to introspect ctypes derived 
classes?  I have to figure out whether things are derived from Structure, 
Union, Array etc. through some ugly indirect methods, and have no idea 

Rob Gaddi, Highland Technology -- www.highlandtechnology.com
Email address domain is currently out of order.  See above to fix.