How does Rounding in Python work?

Question

I'm a bit confused about how rounding works in Python.
Can anyone explain why Python is behaving this way?

Example:

>>> round(0.05,1) # this makes sense 0.1 >>> round(0.15,1) # this doesn't make sense! Why is the result not 0.2? 0.1 

And the same for:

 >>> round(0.25,1) # this makes sense 0.3 >>> round(0.35,1) # in my opinion, should be 0.4 but evaluates to 0.3 0.3 

Edit: In general, it is likely that Python is rounded rather than rounded. So I have to understand that the only "abnormal" thing that can happen is that Python is rounded up? Or can it also round up “abnormally” because of how it is stored? (I did not find a case where Python was rounded, when I expected it to round)

Answer 1

It is actually by design. From the Pythons documentation:

The behavior of round() for floats may be unexpected: for example, round(2.675, 2) gives 2.67 instead of the expected 2.68. This is not a mistake : it is the result of the fact that most decimal fractions cannot be represented exactly as floats.

Answer 2

It seems to me that you need a decimal module:

 from decimal import * x = Decimal('0.15') print x.quantize(Decimal('0.1'), rounding=ROUND_HALF_UP) 

Output:

 0.2 

Answer 3

As an example, here you can round:

 >>> round(0.0499999999999999999,1) 0.1 

In this case, 17 "9" is the minimum number causing this behavior. This is because the internal view is 0.0499999999999999999 0.05000000000000000277555756156289135105907917022705078125 .