Mark Dickinson added the comment:
Huan,
This isn't a bug: see the earlier comments from Zachary Ware on this issue for
explanations. When you compute `rounded(1.45, 0.1)`, you convert the *float*
1.45 to a Decimal instance. Thanks to the What You See Is Not What You Get
nature of binary floating point, the actual value stored for 1.45 is:
1.4499999999999999555910790149937383830547332763671875
Conversion from float to Decimal is exact, so the Decimal value you're working
with is also a touch under 1.45:
>>> from decimal import Decimal
>>> Decimal(1.45)
Decimal('1.4499999999999999555910790149937383830547332763671875')
And so it correctly rounds down to `1.4`.
----------
nosy: +mark.dickinson
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<http://bugs.python.org/issue24827>
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