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pythonroundingformat-string

Format String Rounding Bug

Is it a bug or expected behavior to have rounding behave differently depending on the number of visible decimal places?

Demo via Interactive Interpreter

Python 3.8.10
>>> a = 1.555
>>> f'{a:.0f}'
'2'
>>> f'{a:.1f}'
'1.6'
>>> f'{a:.2f}'
'1.55'
>>> f'{a:.3f}'
'1.555'
Solution

  • One way to make it accurate is to use Decimal:

    But, you could see Decimal(1.555) will output next:

    >>> Decimal(1.555)
Decimal('1.5549999999999999378275106209912337362766265869140625')

    Above could explain why we get the 1.55, as the 3rd number is 4.

    To fix that, we should change float to str as float is not accurate.

    C:\Windows\System32>python
Python 3.9.9 (tags/v3.9.9:ccb0e6a, Nov 15 2021, 18:08:50) [MSC v.1929 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> from decimal import Decimal
>>> a = "1.555"
>>> float(Decimal(a).quantize(Decimal("0")))
2.0
>>> float(Decimal(a).quantize(Decimal("0.1")))
1.6
>>> float(Decimal(a).quantize(Decimal("0.01")))
1.56
>>> float(Decimal(a).quantize(Decimal("0.001")))
1.555
>>>

    So, when you need accurate, consider to using Decimal.