CODEWITHSUNDEEP
pythondecimalprecision
hellouniverse
hellouniverse

Reputation: 123

getcontext().prec not working as expected

from decimal import *

getcontext().prec = 8
print(getcontext(),"\n")

x_amount = Decimal(0.025)
y_amount = Decimal(0.005)

test3 = x_amount - y_amount

print("test3",test3)

Output:

Context(prec=8, rounding=ROUND_HALF_EVEN, Emin=-999999, Emax=999999, capitals=1, clamp=0, flags=[], traps=[InvalidOperation, DivisionByZero, Overflow]) 


test3 0.020000000

Why does this return value of 'test3' up to 9 decimal places if the precision is set to 8 according to example mentioned here?

And it changes to 3 decimal places if I replace Line 6 and 7 in the above code with:

x_amount = Decimal('0.025')
y_amount = Decimal('0.005')

I am using decimals in a financial application and find it very confusing about conversions, operations, definition, precision etc. Is there a link which I can refer to know the details of using decimals in python?

Upvotes: 1

Views: 1841

Answers (1)

ShadowRanger
ShadowRanger

Reputation: 155516

Your result is correct. prec says how many digits to keep starting from the most significant non-zero digit. So in your result:

test3 0.020000000
         ^^^^^^^^

the digits pointed to by carets are the expected eight digits covered by the precision. The reason you get all of them is that using float to initialize Decimal is always wrong. Never initialize a Decimal from a float; it just reproduces the inaccuracy of float in the Decimal (try printing x_value and y_value to see the garbage). Passing a str is the only safe way to do this.

When you do this with str, the individual Decimals "know" the last digit of meaningful precision they possess, and it only goes to the third decimal place, so the result doesn't include precision beyond that point. If you want prec capping to kick in, try initializing one of the arguments with more precision than prec, and the result will be rounded to prec.

Upvotes: 2

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