Lessons · Python · comparing decimals

Decimals that are almost equal

0.1 + 0.2 is 0.30000000000000004, so compare floats with a tolerance, never with ==.

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What it is for

Money totals, measurements, percentages: any sum of decimals drifts by a hair. A test of == that fails on a Tuesday because of rounding is a classic bug.

How to think about it

Ask: are these values the result of arithmetic? If so, compare abs(a - b) < small_number, or use math.isclose. For money, work in integer cents instead.

Worked example

a = 0.1 + 0.2

0.30000000000000004 in binary floating point.

print(a == 0.3)

False. The bug.

print(abs(a - 0.3) < 1e-9)

True. Close enough, and stated as such.

import math; print(math.isclose(a, 0.3))

The standard tool for exactly this.

Your turn

Check two totals agree to within a cent.

same = abs(total_a - total_b) <

The trap

Fixing it by rounding both sides. round(2.675, 2) gives 2.67, not 2.68, for the same reason. Tolerance, not rounding.

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