Mark Dickinson added the comment:
This is working as designed, though admittedly the cause of the unexpected
results is non-obvious.
In Python 2, there's no way to implement `round` for a general type: instead,
the `round` function converts its *input* to a Python (binary) float, and then
rounds the resulting value. So you're effectively doing:
>>> from decimal import Decimal
>>> round(float(Decimal('167.75')), 1)
167.8
>>> round(float(Decimal('167.85')), 1)
167.8
>>> round(float(Decimal('167.95')), 1)
167.9
And the results above are explainable in terms of rounding the corresponding
floating-point value: the closest exactly representable binary float to 167.85
is 167.849999999999994315658113919198513031005859375, which is a touch less
than the halfway mark, so rounds down. Similarly, the closest exactly
representable float to 167.95 is
167.94999999999998863131622783839702606201171875, which again rounds down.
167.75 is already exactly representable as a float, so there you get the
expected result.
In Python 3, `round` rounds the Decimal object directly without converting to
float first, so you shouldn't see the above problems. In Python 2, you'll need
to stick to your quantize approach.
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue21929>
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