[issue31684] Scientific formatting of decimal 0 different from float 0

[Aaron Meurer]

Aaron Meurer <asmeurer at gmail.com> 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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