Floating point "g" format not stripping trailing zeros

[Ian Kelly]

On Wed, Feb 11, 2015 at 2:48 PM, Mark Lawrence <breamoreboy at yahoo.co.uk> wrote:
> On 11/02/2015 20:02, Hrvoje Nikšić wrote:
>>
>> According to the documentation of the "g" floating-point format,
>> trailing zeros should be stripped from the resulting string:
>>
>> """
>> General format. For a given precision p >= 1, this rounds the number
>> to p significant digits and then formats the result in either
>> fixed-point format or in scientific notation, depending on its
>> magnitude.[...]
>> In both cases insignificant trailing zeros are removed from the
>> significand, and the decimal point is also removed if there are no
>> remaining digits following it.
>> """
>>
>> However, in some cases, the trailing zeros apparently remain:
>>
>>>>> from decimal import Decimal as D
>>>>> x = D(1)/D(999)
>>>>> '{:.15g}'.format(x)
>>
>> '0.00100100100100100'
>>
>> For floats, the trailing zeros are removed:
>>
>>>>> '{:.15g}'.format(1. / 999)
>>
>> '0.001001001001001'
>>
>> This behavior is present in both 2.7.8 and 3.4.1. Is this a bug in the
>> formatting of Decimals?
>>
>
> I'd say it's a bug.  P is 15, you've got 17 digits after the decimal place
> and two of those are insignificant trailing zeros.

Actually it's the float version that doesn't match the documentation.
In the decimal version, sure there are 17 digits after the decimal
place there, but the first two -- which are leading zeroes -- would
not normally be considered significant. The float version OTOH is only
giving you 13 significant digits when 15 were requested.

For illustration, compare the same formatting with the scale ramped up
to make it switch to exponential notation:

>>> '{:.15g}'.format(D(1) / D(999000000))
'1.00100100100100e-9'
>>> '{:.15g}'.format(1./999000000)
'1.001001001001e-09'