[Mark Dickinson]
- string to float *and* float to string conversions are both guaranteed
correctly rounded in 3.x: David Gay's code implements the conversion
in both directions, and having correctly rounded string -> float
conversions is essential to ensure that eval(repr(x)) recovers x exactly.
IMO, that is so important that it should be considered a bug fix.
Recently, I lost a lot of time in a discussion about a piece of mathematical
code returning a wrong answer. The problem wasn't the Python code;
instead, it was the str to float conversion (that we inherit from libc) giving
the wrong answer. The code worked correctly under Py3.1 but not
Py2.6.
IDLE 2.6.2
float('-30000000000000000477086733292797541405082425691835844169343982673243516316511143182440409957570445312.00000000000000000009000000000000000258695827614150149965556917147323765827915398318737061345018446445465087890625')
-2.9999999999999999e+100
Python 3.1.1 (r311:74483, Aug 17 2009, 17:02:12) [MSC v.1500 32 bit (Intel)] on
win32
Type "copyright", "credits" or "license()" for more information.
float('-30000000000000000477086733292797541405082425691835844169343982673243516316511143182440409957570445312.00000000000000000009000000000000000258695827614150149965556917147323765827915398318737061345018446445465087890625')
-3.0000000000000002e+100
- the repr of round(x, n) really does have at most n digits after the
point, giving the semi-illusion that x really has been rounded exactly,
and eliminating one of the most common user complaints about the
round function.
This is also an important improvement and makes round() do what
people expect.
- side effects like finding that float(x) rounds correctly for
Decimal instances x.
This is also important because other decimal calculations can
often be done exactly and it's a bummer to have an accurate
result thrown off by an incorrect rounding to float.
- the float <-> string conversions are under our control, so any bugs
found can be fixed in the Python source. There's no shortage of
conversion bugs in the wild, and certainly bugs have been observed in
OS X, Linux and Windows. (The ones I found in OS X 10.5 have
been fixed in OS X 10.6, though.)
Nice win.
Thoughts?
+1
I've worked with the 3.1 float reprs for a while and have been delighted.
It was a great improvement.
Raymond
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