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How about just punting signed conversion.  I don't think
two's complement makes much sense for arbitrary precision
longs.  Have some separate representation for negative longs
if needed.  If you call hex() on a large negative number,
you get a hex string with a leading minus sign.  For base
256, you can't reserve a char like that, so I guess you have
to just throw an error if someone tries to convert a
negative long to a string.  If you want a representation for
signed longs, ASN1 DER is probably an ok choice.  I agree
with Guido that the binascii module is a good place to put
such a function.  Twos complement can work if you specify a
fixed precision, but that sure complicates what this started
out as.