Best way to convert fractions.Fraction to decimal.Decimal?

Question

In Python, the fractions.Fraction and decimal.Decimal standard library classes exist to help keep arithmetic with rational numbers precise. For the unfamiliar, an example of where it helps:

>>> 1 / 10 * 3
0.30000000000000004
>>> decimal.Decimal('1') / 10 * 3
Decimal('0.3')
>>> fractions.Fraction('1') / 10 * 3
Fraction(3, 10)

My question is, if I have a Fraction, what's the best way to convert it to a Decimal?

Unfortunately the obvious solution doesn't work:

>>> decimal.Decimal(fractions.Fraction(3, 10))
Traceback (most recent call last):
  ...
TypeError: conversion from Fraction to Decimal is not supported

Right now I'm using this code:

>>> decimal.Decimal(float(fractions.Fraction(3, 10)))
Decimal('0.299999999999999988897769753748434595763683319091796875')

Now, when I actually output this value, any amount of rounding will convert it to 0.3, and I only do this conversion immediately before output (all the core math is done with Fraction). Still, it seems a bit silly to me that I can't get a Decimal('0.3') from a Fraction(3, 10). Any help would be appreciated!

Answer 1

How about leaving the division of the fraction to Decimal() itself?

def decimal_from_fraction(frac):
    return frac.numerator / decimal.Decimal(frac.denominator)

This is what Fraction.__float__() does (simply divide the numerator by the denominator), but by turning at least one of the two values into a Decimal object you get to control the output.

This lets you use the decimal context:

>>> decimal_from_fraction(fractions.Fraction(3, 10))
Decimal('0.3')
>>> decimal_from_fraction(fractions.Fraction(1, 55))
Decimal('0.01818181818181818181818181818')
>>> with decimal.localcontext() as ctx:
...    ctx.prec = 4
...    decimal_from_fraction(fractions.Fraction(1, 55))
...
Decimal('0.01818')