请熟悉python的指导一下

[kernelshell]

def export_xml_etree(self,filename):

root = xml.etree.ElementTree.Element("incidents")

for incident in self.values():

element = xml.etree.ElementTree.Element("incident",report_id = incident.report_id,date = incident.date.isoformat(),aircraft_id = \

incident.aircraft_id,aircraft_type = incident.aircraft_type,pilot_percent_hourse_on_type=str(incident.pilot_prcent_hours_on_type),pilot_total_hours=str \

(incident.pilot_total_hours),midair=str(int(incident.midair)))

airport = xml.etree.ElementTree.SubElement(element,"airport")

airport.text = incident.airport.strip()

narrative = xml.etree.ElementTree.SubElement(element,"narrative")

narrative.text = incident.narrative.strip()

root.append(element)

tree = xml.etree.ElementTree.ElementTree(root)

try:

tree.write(filename,"UTF-8")

except EnvironmentError as err:
print("{0} : import error:{1}".format(os.path.basename(sys.argv[0]),err))

return True

是用pdb调试，但是显示：-> def export_xml_etree(self,filename):
（pdb）s
--Return--
（pdb）l
（EOF）
我的疑问是为什么使用“s"命令，进不去这个函数呢？我用l察看，却显示EOF，到达了文件的末尾？这是怎么回事？为什么单步调试不了呢？谢谢！