Python Decimal vs C# decimal precision [duplicate]

Question

This question already has an answer here:

• Why python decimal.Decimal precision differs with equable args? 2 answers

I know this has been asked numerous times and I've come across many blogs and SO answers but this one's making me pull my hair out. I just want to multiply a two decimal number by 100 so I get rid of its decimals:

>>> 4321.90 * 100
432189.99999999994
>>> Decimal(4321.90) * Decimal(100)
Decimal('432189.9999999999636202119291')

I'm scared to use rounding for such seemingly trivial operation. Would it be safe? What if the precision problem plays tricks on me and the result is close to xxx.5? Can that happen? I do understand the problem at the binary level, but I come from C# and I don't have that problem with .Net's decimal type:

decimal x = 4321.90m;
decimal y = 100m;
Console.WriteLine(x * y);
432190,00

I thought Python's decimal module was supposed to fix that. I'm about to convert the initial value to string and do the math with string manipulations, and I feel bad about it...

Answer 1

The main reason it fails with Python is because 4321.90 is interpreted as float (you lose precision at that point) and then casted to Decimal at runtime. With C# 4321.90m is interpreted as decimal to begin with. Python simply doesn't support decimals as a built-in structure.

But there's an easy way to fix that with Python. Simply use strings:

>>> Decimal('4321.90') * Decimal('100')
Decimal('432190.00')

Answer 2

I'm about to convert the initial value to string

Yes! (but don't do it by calling str - use a string literal)

and do the math with string manipulations

No!

When hardcoding a decimal value into your source code, you should initialize it from a string literal, not a float literal. With 4321.90, floating-point rounding has already occurred, and building a Decimal won't undo that. With "4321.90", Decimal has the original text you wrote available to perform an exact initialization:

Decimal('4321.90')

Answer 3

Floating point inaccuracy again.

Decimal(number) doesn't change a thing: the value is modified before it hits Decimal.

You can avoid that by passing strings to Decimal, though:

Decimal("4321.90") * Decimal("100")

result:

Decimal('432190.00')

(so Decimal handles the floating point numbers without using the floating point registers & operations at all)