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 + 0.20.30000000000000004 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) <
Solve one with the tests running
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.
Now you try
Next in Python
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