[Python-Dev] Two proposed changes to float formatting
python at rcn.com
Mon Apr 27 01:35:43 CEST 2009
>> it would make more sense to me if it
>> stayed fixed at 56 sig digits for numbers larger than 1e50.
> So I agree with this, even if the default # of sig digits were less.
Several reasons to accept Mark's proposal:
* It matches what C does and many languages tend to copy the
C standards with respect to format codes. Matching other
languages helps in porting code, copying algorithms, and mentally
switching back and forth when working in multiple languages.
* When a programmer has chosen %f, that means that they have
consciously rejected choosing %e or %g. It is generally best to
have the code do what the programmer asked for ;-)
* Code that tested well with 1e47, 1e48, 1e49, and 1e50
suddenly shifts behavior with 1e51. Behavior shifts like that
are bug bait.
* The 56 significant digits may be rooted in the longest
decimal expansion of a 53 bit float. For example,
len(str(Decimal.from_float(.1))) is 57 including the leading
zero. But not all machines (now, in the past, or in the future)
use 53 bits for the significand.
* Use of exponents is common but not universal. Some converters
for SQL specs like Decimal(10,80) may not recognize the
e-notation. The xmlrpc spec only accepts decimal expansions
not %e notation.
* The programmer needs to have some way to spell-out a
decimal expansion when needed. Currently, %f is the only way.
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