Taking the reciprocal of a Fraction

Question

I want to take the reciprocal of Fraction in python, and do so in-place. The Fraction class does not provide a method for doing so (in my knowledge). I tried to just swap numerator and denominator:

f = Fraction(2, 3)
f.numerator, f.denominator = f.denominator, f.numerator

but it resulted in an AttributeError: can't set attribute .

I've also used the method of just constructing an new Fraction :

f = Fraction(2, 3)
f = Fraction(f.denominator, f.numerator)

which does in fact work, but creating a new object and deleting the old one doesn't seem very 'pythonic', ie unnecessarily complex. Is there a better way to take the reciprocal?

Answer 1

from fractions import Fraction

spam = Fraction(2, 3)
eggs = spam ** -1
print(repr(eggs))

output

Fraction(3, 2)

EDIT: As suggested in comments by @martineau 1 / spam also works:

from fractions import Fraction

spam = Fraction(2, 3)
eggs = 1 / spam 
print(repr(eggs))

Answer 2

i think f**-1 this will do the job

Answer 3

Fractions are immutable like many (all?) Python numbers, which is why you can't change their attributes. The workaround is to define a function that creates and returns a new instances of the class based on the attributes of the one passed to it. Two ways of doing this are shown below.

from fractions import Fraction


def reciprocal(f):
    """ Return reciprocal of argument (an instance of Fraction). """
    return Fraction(f.denominator, f.numerator)

# Alternative implementation.
reciprocal = lambda f: Fraction(f.denominator, f.numerator)


f = Fraction(2, 3)
print(f)  # -> 2/3
print(repr(f))  # -> Fraction(2, 3)
r = reciprocal(f)
print(r)  # -> 3/2
print(repr(r))  # -> Fraction(3, 2)