Check if a number is rational in Python, for a given fp accuracy

Question

I would like to know a good way of checking if a number x is a rational (two integers n,m exist so that x=n/m) in python.

In Mathematica, this is done by the functio

Answer 1

Python uses floating-point representation rather than rational numbers. Take a look at the standard library fractions module for some details about rational numbers.

Observe, for example, this, to see why it goes wrong:

>>> from fractions import Fraction
>>> 1.1  # Uh oh.
1.1000000000000001
>>> Fraction(1.1)  # Will only work in >= Python 2.7, anyway.
Fraction(2476979795053773, 2251799813685248)
>>> Fraction(*1.1.as_integer_ratio())  # Python 2.6 compatible
Fraction(2476979795053773, 2251799813685248)

(Oh, you want to see a case where it works?)

>>> Fraction('1.1')
Fraction(11, 10)