def __init__(self, istep, sdat):
self.istep = istep<EOL>self.sdat = sdat<EOL>self.fields = _Fields(self, phyvars.FIELD, phyvars.FIELD_EXTRA,<EOL>phyvars.FIELD_FILES, phyvars.FIELD_FILES_H5)<EOL>self.sfields = _Fields(self, phyvars.SFIELD, [],<EOL>phyvars.SFIELD_FILES, [])<EOL>self.tracers = _Tracers(self)<EOL>self._isnap = UNDETERMINED<EOL>
Initialization of instances:

        Args:
            istep (int): the index of the time step that the instance
                represents.
            sdat (:class:`~stagpy.stagyydata.StagyyData`): the StagyyData
                instance owning the :class:`Step` instance.

        Attributes:
            istep (int): the index of the time step that the instance
                represents.
            sdat (:class:`~stagpy.stagyydata.StagyyData`): the StagyyData
                instance owning the :class:`Step` instance.
            fields (:class:`_Fields`): fields available at this time step.
            sfields (:class:`_Fields`): surface fields available at this time
                step.
            tracers (:class:`_Tracers`): tracers available at this time step.