import ctypes

class Bad(ctypes.Structure):
    _pack_ = 1
    _fields_ = [
                ( "a0"  , ctypes.c_uint8 ,1   ),
                 ("a1" , ctypes.c_uint8 ,1   ),
                 ("a2" , ctypes.c_uint8 ,1   ),
                 ("a3" , ctypes.c_uint8 ,1   ),
                 ("a4" , ctypes.c_uint8 ,1   ),
                 ("a5" , ctypes.c_uint8 ,1   ),
                 ("a6" , ctypes.c_uint8 ,1   ),
                 ("a7" , ctypes.c_uint8 ,1   ),

                 ("b0" , ctypes.c_uint16 ,4  ),
                 ("b1" , ctypes.c_uint16 ,12 ),
    ]
class GoodA(ctypes.Structure):
    _pack_ = 1
    _fields_ = [
                ( "a0"  , ctypes.c_uint8 ,1   ),
                 ("a1" , ctypes.c_uint8 ,1   ),
                 ("a2" , ctypes.c_uint8 ,1   ),
                 ("a3" , ctypes.c_uint8 ,1   ),
                 ("a4" , ctypes.c_uint8 ,1   ),
                 ("a5" , ctypes.c_uint8 ,1   ),
                 ("a6" , ctypes.c_uint8 ,1   ),
                 ("a7" , ctypes.c_uint8 ,1   ),
    ]

class Good(ctypes.Structure):
    _pack_ = 1
    _fields_ = [("a", GoodA),
                ("b0" , ctypes.c_uint16 ,4  ),
                ("b1" , ctypes.c_uint16 ,12 ),
                ]
def printCTypeOffsets(cStruct):
    for f in cStruct._fields_:
        ft = getattr(cStruct, f[0])
        print ft.offset,hex(ft.size),f[0]

print "Size is ", ctypes.sizeof(Bad), "Expected 3"
printCTypeOffsets(Bad)

print "Size is ", ctypes.sizeof(Good), "Expected 3"
printCTypeOffsets(Good)