def _open(self, filename, mode):
if mode not in 'aw':
raise ValueError('mode must be "w" or "a".')
if self.master:
self.backend = affopen(filename, mode, tag='ASE-Trajectory')
if len(self.backend) > 0:
r = affopen(filename)
self.numbers = r.numbers
self.pbc = r.pbc
else:
self.backend = DummyWriter()
