'Returns the value denoting "no data" for this variable.
If this variable does not have a missing/fill value, returns None.
If both _FillValue and missing_value are given, give precedence to
_FillValue. The netCDF standard gives special meaning to _FillValue;
missing_value is  just used for compatibility with old datasets.'

def _get_missing_value(self):

    if ('_FillValue' in self._attributes):
        missing_value = self._attributes['_FillValue']
    elif ('missing_value' in self._attributes):
        missing_value = self._attributes['missing_value']
    else:
        missing_value = None
    return missing_value
