<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def source_file(self):
 """ When using open, returns the filename used """

        if self._filename is None:
            self._filename = self._as_in_memory_geotiff()._filename
        return self._filename