On Mon, Mar 13, 2017 at 12:57 PM Eric Wieser <wieser.eric+nu...@gmail.com> wrote:
> > `float(repr(a)) == a` is guaranteed for Python `float` > > And `np.float16(repr(a)) == a` is guaranteed for `np.float16`(and the same > is true up to `float128`, which can be platform-dependent). Your code > doesn't work because you're deserializing to a higher precision format than > you serialized to. > I would hesitate to make this guarantee - certainly for old versions of numpy, np.float128(repr(x))!=x in many cases. I submitted a patch, now accepted, that probably accomplishes this on most systems (in fact this is now in the test suite) but if you are using a version of numpy that is a couple of years old, there is no way to convert long doubles to human-readable or back that doesn't lose precision. To repeat: only in recent versions of numpy can long doubles be converted to human-readable and back without passing through doubles. It is still not possible to use % or format() on them without discarding all precision beyond doubles. If you actually need long doubles (and if you don't, why use them?) make sure your application includes a test for this ability. I recommend checking repr(1+np.finfo(np.longdouble).eps). Anne P.S. You can write (I have) a short piece of cython code that will reliably repr and back long doubles, but on old versions of numpy it's just not possible from within python. -A
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