Returns the fraction with additional whitespace.

def str_fraction(self):
        """Returns the fraction with additional whitespace."""
        if self._eta.undefined:
            return None

        # Determine denominator and its unit.
        unit_denominator, unit = self._unit_class(self.denominator).auto
        formatter = '%d' if unit_denominator == self.denominator else '%0.2f'
        denominator = locale.format(formatter, unit_denominator, grouping=True)

        # Determine numerator.
        unit_numerator = getattr(self._unit_class(self.numerator), unit)
        if self.done:
            rounded_numerator = unit_numerator
        else:
            rounded_numerator = float(Decimal(str(unit_numerator)).quantize(Decimal('.01'), rounding=ROUND_DOWN))
        numerator = locale.format(formatter, rounded_numerator, grouping=True).rjust(len(denominator))

        return '{0}/{1} {2}'.format(numerator, denominator, unit)