def __init__(self, f, sub=slice(None)):
SlicedReader.__init__(self, f, sub)<EOL>self._secfile = SectionFile(self._f)<EOL>self._read_header()<EOL>
Arguments:
 | ``f``  --  a filename or a file-like object
 | ``sub``  --  an optional slice object to select a subset of time
                frames

The number of atoms is read immediately and is available as
``self.num_atoms``.