Can someone please say what's wrong with my code?

Question

https://code.sololearn.com/cpeJoaxm7u1Z/?ref=app

Answer 1

I think the error comes from not distinguishing between the methods vt and a (marked as private) _vt attribute

Answer 2

In the example based on your code, I renamed the attribute _vt (marked as private to discourage user from directly accessing it)
https://code.sololearn.com/cgOjJD5Jrdb1/?ref=app

Answer 3

Not solved yet

Answer 4

Can someone please rewrite it?