python3 self动态调用?

if self.idx == 0:
    self.line_0.start = v
    self.line_follow.start = v
    self.idx += 1
elif self.idx == 1:
    self.line_0.end = v
    self.line_follow.start = None
    self.idx += 1
elif self.idx == ...:
...

以上代码可以简化为

i = self.idx/2
i_f = floor(i)
k = 'start'
if i > i_f:
    k = 'end'
    self.line_follow.start = None
else:
    self.line_follow.start = v
self['line_' + str(i_f)][k] = v
self.idx += 1

但 self['line_' + str(i_f)] 不能这么用，有什么办法能动态调用吗？