import ctypes

class MyUnion(ctypes.Union):
    _fields_ = [('d', ctypes.c_double), ('i', ctypes.c_int64)] \
               + [("x{}".format(i), ctypes.c_double) for i in range(2,13)]

lib = ctypes.CDLL('./libfoo.so')
foo = lib.foo
foo.argtypes = [MyUnion]
foo.restype = None

ind = MyUnion()
ind.d = 3.1415
foo(ind)

ini = MyUnion()
ini.i = 42
foo(ini)