I just discovered that struct packs pointers from objects with an
__index__() method. Is that intentional?

>>> import struct
>>> class IDX(object):
...     def __init__(self, value):
...         self.value = value
...     def __index__(self):
...          return self.value
... 
>>> struct.pack('P', IDX(9))
b'\t\x00\x00\x00\x00\x00\x00\x00'
>>>