CODEWITHSUNDEEP
pythonfloating-pointroundingprecisionfloating-accuracy
Jonas Kaufmann
Jonas Kaufmann

Reputation: 1817

How does Rounding in Python work?

I am a bit confused about how rounding in Python works.
Could someone please explain why Python behaves like this?

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 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: So in general, there is a possibility that Python rounds down instead of rounding up. So am I to understand that the only "abnormal" thing that can happen is that Python rounds down? Or may it also get rounded up "abnormally" due to how it is stored? (I haven't found a case where Python rounded up when I expected it to round down)

Upvotes: 5

Views: 2895

Answers (3)

Idos
Idos

Reputation: 15310

This is actually by design. From Pythons' documentation:

The behavior of round() for floats can be surprising: for example, round(2.675, 2) gives 2.67 instead of the expected 2.68. This is not a bug: it’s a result of the fact that most decimal fractions can’t be represented exactly as a float.

Upvotes: 12

Galax
Galax

Reputation: 1431

It sounds to me like you need the decimal module:

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

Output:

0.2

Upvotes: 8

gariepy
gariepy

Reputation: 3674

As an example, here is a case where you get rounding up:

>>> round(0.0499999999999999999,1)
0.1

In this case, 17 "9"s are the minimum number to cause this behavior. This is because the internal representation of 0.0499999999999999999 is 0.05000000000000000277555756156289135105907917022705078125.

Upvotes: 4

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