def fraction(target, numerator, denominator):
    """
    Calculates the ratio between two values

    Parameters
    ----------
    target : OpenPNM Object
        The object which this model is associated with. This controls the
        length of the calculated array, and also provides access to other
        necessary properties.

    numerator : string
        Dictionary key pointing the numerator values

    denominator : string
        Dictionary key pointing the denominator values
    """
    x = target[numerator]
    y = target[denominator]
    return x / y
