def read_acceptance_fraction(self, temps=None, walkers=None):
        """Reads the acceptance fraction.

        Parameters
        -----------
        temps : (list of) int, optional
            The temperature index (or a list of indices) to retrieve. If None,
            acfs from all temperatures and all walkers will be retrieved.
        walkers : (list of) int, optional
            The walker index (or a list of indices) to retrieve. If None,
            samples from all walkers will be obtained.

        Returns
        -------
        array
            Array of acceptance fractions with shape (requested temps,
            requested walkers).
        """
        group = self.sampler_group + '/acceptance_fraction'
        if walkers is None:
            wmask = numpy.ones(self.nwalkers, dtype=bool)
        else:
            wmask = numpy.zeros(self.nwalkers, dtype=bool)
            wmask[walkers] = True
        if temps is None:
            tmask = numpy.ones(self.ntemps, dtype=bool)
        else:
            tmask = numpy.zeros(self.ntemps, dtype=bool)
            tmask[temps] = True
        return self[group][:][numpy.ix_(tmask, wmask)]