A struct for 2.4 that supports float's inf and nan?

Joshua J. Kugler joshua at eeinternet.com
Thu Sep 20 21:45:13 CEST 2007

On Thursday 20 September 2007 11:19, Paul Hankin wrote:

>> I suppose I could get the relevant source from the 2.5 source and compile
>> it as a custom package, but that wouldn't be very transparent for my
>> users, and would probably be getting in way over my head. :)
>> Ideas?  Suggestions?
> Here's a wrapped pack:
> import struct
> pack_double_workaround = {
>   'inf': struct.pack('Q', 0x7ff0000000000000L),
>   '-inf': struct.pack('Q', 0xfff0000000000000L),
>   'nan': struct.pack('Q', 0x7ff8000000000000L)
> }
> def pack_one_safe(f, a):
>   if f == 'd' and str(f) in pack_double_workaround:
>       return pack_double_workaround[str(f)]
>   return struct.pack(f, a)
> def pack(fmt, *args):
>   return ''.join(pack_one_safe(f, a) for f, a in zip(fmt, args))
> Unpacking is similar: unpack doubles with 'Q' and test the
> long for equality with +-inf, and find nan's by checking bits
> 52 to 62. If the number's ok, unpack again using 'd'.
> You can get python values for nan, -inf and inf by using
> float('nan'), float('-inf'), float('inf').
> I've not been able to properly test this, as struct seems
> to work fine in Python 2.3 and 2.4 on MacOS X.

Thanks for the ideas, Paul!  I came up with something that works for me, but
this has a few ideas that I'm going to implement in my wrapper to make for
cleaner code.

As to testing it on MacOS X: yeah, it can be a somewhat system-dependent
problem, so may not show up on all architectures.

Thanks for the tips!


Joshua Kugler
Lead System Admin -- Senior Programmer
PGP Key: http://pgp.mit.edu/  ID 0xDB26D7CE

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