max / min / smallest float value on Python 2.5

[Mark Dickinson]

On Feb 7, 12:52 am, duncan smith <buzz... at urubu.freeserve.co.uk>
wrote:
> import platform
> if platform.architecture()[0].startswith('64'):
>      TINY = 2.2250738585072014e-308
> else:
>      TINY = 1.1754943508222875e-38

As Christian said, whether you're using 32-bit or 64-bit shouldn't
make a difference here.  Just use the first TINY value you give.

> I'm not 100% sure how reliable this will be across platforms.  Any ideas
> about the cleanest, reliable way of uncovering this type of information?

In practice, it's safe to assume that your 2.225....e-308 value is
reliable across platforms.  That value is the one that's appropriate
for the IEEE 754 binary64 format, and it's difficult these days to
find CPython running on a machine that uses any other format for C
doubles (and hence for Python floats).

The smallest positive *normal* number representable in IEEE 754
binary64 is exactly 2**-1022 (or approximately
2.2250738585072014e-308).  The smallest positive *subnormal* number
representable is exactly 2**-1074, or approximately
'4.9406564584124654e-324'.  (Subnormals have fewer bits of precision
than normal numbers;  it's the presence of subnormals that allows for
'gradual underflow'.)  Some machines can/will treat subnormal numbers
specially for speed reasons, either flushing a subnormal result of a
floating-point operation to 0, or replacing subnormal inputs to an
floating-point operation with 0, or both.  So for maximal portability,
and to avoid numerical problems, it's best to avoid the subnormal
region.

> The precise issue is that I'm supplying a default value of
> 2.2250738585072014e-308 for a parameter (finishing temperature for a
> simulated annealing algorithm) in an application.  I develop on
> Ubuntu64, but (I am told) it's too small a value when run on a Win32
> server.  I assume it's being interpreted as zero and raising an
> exception.

This is a bit surprising.  What's the precise form of the error you
get?  Do you still get the same error if you replace your TINY value
by something fractionally larger?  (E.g., 2.23e-308.)

--
Mark