pyinterval with trvial floating point example doesn't seem to be working

There are probably hundreds of questions on here about floating point rou开发者_JAVA百科nding errors, and the solution is supposed to be interval arithmetic which helps keep track of them. As I'd like to experiment with this, I tried installing pyinterval.

Unfortunately, it doesn't seem to be working. For this trivial case of floating point error, I'd expect an interval with 2 numbers, one higher and one lower than the actual answer (0.001):

>>> from interval import interval
>>> interval(1.001)-1
interval([0.0009999999999998899])

After some head scratching, my best guess was that the number 1.001 is being converted to a single floating point value before being passed to the interval module, so I tried using the floating point fpu.up and fpu.down functions to force the conversion:

>>> from interval import fpu
>>> fpu.down(lambda : float("1.001")-1)
0.0009999999999998899
>>> fpu.up(lambda : float("1.001")-1)
0.0009999999999998899

Any idea what is wrong? Or have I got the wrong end of the stick on how this is supposed to work?

I'm using an x86 mac with "Python 2.7.1 (r271:86832, Jan 1 2011, 22:28:41) [GCC 4.2.1 (Apple Inc. build 5664)] on darwin"

It looks like the conversion of strings to floats is not respecting the rounding, but if I do it manually by entering 1.001 as 1001/1000, I get the sort of thing I am expecting:

>>> (interval[1001]/1000)-1
interval([0.0009999999999996678, 0.001000000000000112])