On 12/02/2015 23:46, Ian Kelly wrote:
On Thu, Feb 12, 2015 at 1:23 PM, Hrvoje Nikšić <hnik...@gmail.com> wrote:
from decimal import Decimal as D
x = D(1)/D(999)
'{:.15g}'.format(x)
'0.00100100100100100'
[...]
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.
{:.15g} is supposed to give 15 digits of precision, but with trailing
zeros removed.
The doc says with "insignificant" trailing zeros removed, not all
trailing zeros.
For example, '{:.15g}'.format(Decimal('0.5')) should
yield '0.5', not '0.500000000000000' -- and, it indeed does. It is
only for some numbers that trailing zeros are not removed, which looks
like a bug. The behavior of floats matches both the documentation and
other languages using the 'g' decimal format, such as C.
Ah, I see now what's going on here. With floats, there is really no
notion of significant digits. The values 0.5 and 0.50000 are
completely equivalent. With decimals, that's not exactly true; if you
give the decimal a trailing zero then you are telling it that the zero
is significant.
Decimal('0.5')
Decimal('0.5')
Decimal('0.50000')
Decimal('0.50000')
Decimal('0.5').as_tuple()
DecimalTuple(sign=0, digits=(5,), exponent=-1)
Decimal('0.50000').as_tuple()
DecimalTuple(sign=0, digits=(5, 0, 0, 0, 0), exponent=-5)
These are distinct; the decimal knows how many significant digits you
passed it. As a result, these are also distinct:
'{:.4g}'.format(Decimal('0.5'))
'0.5'
'{:.4g}'.format(Decimal('0.50000'))
'0.5000'
Now what happens in your original example of 1/999? The default
decimal context uses 28 digits of precision, so the result of that
calculation will have 28 significant digits in it.
decimal.getcontext().prec
28
Decimal(1) / Decimal(999)
Decimal('0.001001001001001001001001001001')
When you specify the a precision of 15 in your format string, you're
telling it to take the first 15 of those. It doesn't care that the
last couple of those are zeros, because as far as it's concerned,
those digits are significant.
I still think it's a bug as the 'p' being referred to in the OP's
original message is "The precision is a decimal number indicating how
many digits should be displayed after the decimal point for a floating
point value formatted with 'f' and 'F', or before and after the decimal
point for a floating point value formatted with 'g' or 'G'". In other
words is has nothing to do with the precision of the underlying number.
--
My fellow Pythonistas, ask not what our language can do for you, ask
what you can do for our language.
Mark Lawrence
--
https://mail.python.org/mailman/listinfo/python-list
