On 29/07/2010 22:37, Robert Brewer wrote:
Mark Dickinson wrote:
Now that we've got the short float repr in Python, there's less value
in having float.__str__ truncate to 12 significant digits (as it
currently does). For Python 3.2, I propose making float.__str__ use
the same algorithm as float.__repr__ for its output (and similarly for
complex).
Apart from simplifying the internals a little bit, one nice feature of
this change is that it removes the differences in formatting between
printing a float and printing a container of floats:
l = [1/3, 1/5, 1/7]
print(l)
[0.3333333333333333, 0.2, 0.14285714285714285]
print(l[0], l[1], l[2])
0.333333333333 0.2 0.142857142857
Any thoughts or comments on this?
There's a working patch at http://bugs.python.org/issue9337
Python 2.5.4 (r254:67916, Jan 20 2010, 21:44:03)
float("0.142857142857") * 7
0.99999999999899991
float("0.14285714285714285") * 7
1.0
I've made a number of tools in the past that needed to round-trip a
float through a string and back. I was under the impression that floats
needed 17 decimal digits to avoid losing precision. How does one do that
efficiently if neither str nor repr return 17 digits?
Because every floating point number represents a range of values - and
the new algorithm uses the shortest possible representation within that
range of values that will return the same floating point number.
Mark did an excellent presentation on this at EuroPython and the slides
are online:
http://www.slideshare.net/dickinsm/magical-float-repr
Michael Foord
Robert Brewer
fuman...@aminus.org
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