def METHOD_NAME(self, acceptance_fraction):
    """Write acceptance_fraction data to file.
    Results are written to ``[sampler_group]/acceptance_fraction``; the
    resulting dataset has shape (ntemps, nwalkers).
    Parameters
    -----------
    acceptance_fraction : numpy.ndarray
        Array of acceptance fractions to write. Must have shape
        ntemps x nwalkers.
    """
    # check
    assert acceptance_fraction.shape == (self.ntemps, self.nwalkers), (
        "acceptance fraction must have shape ntemps x nwalker")
    group = self.sampler_group + '/acceptance_fraction'
    try:
        self[group][:] = acceptance_fraction
    except KeyError:
        # dataset doesn't exist yet, create it
        self[group] = acceptance_fraction