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'
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.