最长用到的异常处理的形式：
try:
正常的操作
except:
发生异常，执行这块代码
else:
如果没有异常执行这块代码
finally:
不管如何，最后一定要执行的代码
try:
10/0
except Exception as e:
print ('出错了:', e)
else:
print('ok')
finally:
print('finally')
出错了: division by zero
finally
try:
10/5
except Exception as e:
print ('出错了:', e)
else:
print('ok')
finally:
print('finally')
ok
finally
