Aaron Meurer <[email protected]> added the comment:
I meant that format() destroys information in a decimal in general. Obviously
if you have n digits of precision and format with m < n, then you lose
information.
I also can't help but feel that we're mixing up "trailing zeros" (i.e.,
precision), and "exponent" (magnitude), which should be orthogonal. I'm
assuming that a decimal is represented internally as base*10**exponent. I'm
also assuming that Decimal(0) sets both base and exponent to 0. It doesn't make
sense to me that a string formatting operation that requests a certain number
of digits of precision should change the exponent.
I get that 0.0 is different from 0.00000, but in that case, they should print
differently: as '0.0' and '0.00000'. It seems sly to try to maintain that
through a format operation via the exponent, especially when format *in
general* loses precision information for a decimal anyway (by "format" I mean
format with a set number of digits requested). Especially since that "trick"
only works for exactly one number, zero. If you do
'{:+.30e}'.format(Decimal('1.0000000000000000000')) or
'{:+.10e}'.format(Decimal('1.0000000000000000000')), no such trick is used,
because no such trick can be used. You just lose information.
I'm sure my mental model is off here. I'm used to sympy.Float/mpmath.mpf where
values like 0*2**i are normalized to i = 0 (e.g. mpmath.mpf((0, 0, 20,
0))._mpf_ gives (0, 0, 0, 0)), so this problem never comes up in the code that
I'm used to.
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<https://bugs.python.org/issue31684>
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