CODEWITHSUNDEEP
pythonfloating-pointdecimal
Nicholas
Nicholas

Reputation: 2668

Why bother quantize() a floating-point number given that we have round() function?

I read in a Python book saying that in financial world, sometimes it'd be better using quantize() and decimal module to round off floating-point numbers. An example is given in the book as below,

from decimal import Decimal
price = Decimal('19.99')
tax = Decimal('0.06')
total = price + (price * tax) 
penny = Decimal('0.01')
total.quantize(penny)

But why not

round(19.99+19.99*0.06,2)

When does quantize() outperform round() in terms of numerical accuracy? Anyone can provide an example?

Upvotes: 0

Views: 1289

Answers (1)

Tomas Kulich
Tomas Kulich

Reputation: 15638

I don't think that the main reason for using Decimal (and quantize) is speed: it's accuracy. From the docs:

Decimal numbers can be represented exactly. In contrast, numbers like 1.1 and 2.2 do not have exact representations in binary floating point. End users typically would not expect 1.1 + 2.2 to display as 3.3000000000000003 as it does with binary floating point.

If you are dealing with big numbers and you require the result to be correct up to 1 cent, then the accuracy (with floating point arithmetics) may be an issue.

Upvotes: 2

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