[Python-Dev] Adding DBL_MANTISSA and such to Python

[Edward C. Jones]

Recently I needed some information about the floating point numbers on 
my machine. So I wrote a tiny C99 program with the line

printf("%a\n", DBL_EPSILON);

The answer was "0x1p-52".

A search of comp.lang.python shows that I was not alone. Here are some 
ideas.

1. Add to Python the constants in "float.h" and "limits.h".

2. Add the C99 "%a" format to the "%" operator for strings and allow it 
in floating point literals.

3. Add full "tostring" and "fromstring" capabilities for Python numeric 
types. "tostring(x)" would return a string containing the binary 
representation of x. For example, if x is a Python float, "tostring(x)" 
would have eight characters. "fromstring(s, atype)" does the reserve, so
     fromstring(tostring(x), type(x)) == x

4. Add some functions that process floating point types at a low level. 
I suggest borrowing from C
     (mantissa, exponent) = frexp(x)
where mantissa is a float and exponent is an int. The mantissa can be 
0.0 or 0.5 <= mantissa < 1.0. Also x = mamtissa * 2**exponent. If
x == 0.0, the function returns (0.0, 0). (This is almost a quote from 
Harbison and Steele.)

5. Add the C99 constants and functions involving special floating point 
values: "FP_INFINITE", "FP_NAN", "FP_NORMAL", "FP_SUBNORMAL", "FP_ZERO", 
"fpclassify", "isfinite", "isinf", "isnan", "isnormal", "signbit", 
"copysign", "nan", "nextafter", and "nexttoward". There has been 
controversy about these in the past, but I am in favor of them. The 
documentation should discuss portability.