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In Python 2.7, struct.pack with an integer format can handle non-integers that provide an __int__ method (although this *does* raise a DeprecationWarning).
Python 2.7a4+ (trunk:79659:79661, Apr 3 2010, 11:28:19) 
[GCC 4.2.1 (Apple Inc. build 5646) (dot 1)] on darwin
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>>> from struct import pack
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>>> pack('L', 3.1415)
'\x03\x00\x00\x00\x00\x00\x00\x00'
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This behaviour isn't particularly desirable for floats or Decimal instances, but it's useful for integer-like objects.
In Python 3.x, there's no provision for handling integer-like objects than aren't actually integers.
I propose that in 3.x, struct.pack should try to convert any non-integer to an integer by using its __index__ method, before packing.