[Python-Dev] Proposal: make float.__str__ identical tofloat__repr__ in Python 3.2

[Robert Brewer]

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?


Robert Brewer
fumanchu at aminus.org