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

        Parameters
        -----------
        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 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
        return self[group][wmask]