[Python-Dev] Proposal: make float.str identical tofloat__repr__ in Python 3.2 (original) (raw)

Michael Foord fuzzyman at voidspace.org.uk
Thu Jul 29 23:53:01 CEST 2010

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 fumanchu at aminus.org

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