It would be very helpful to add more information to the
IndexErrors raised by the built-in classes.

Today, these errors look like:

  IndexError: list index out of range

When these occur, I usually have to add print
statements before the error and reproduce the problem
to determine what has gone wrong.  

If the list and tuple classes add the requested index
and the length of the structure, it would help a lot. 
Below is a suggestion:

  IndexError: list index 30 out of range; len=27

I don't think this is bearly needed. This error messages (IndexError:
list index out of range) comes from so long ago that those kind of
changes makes me feel worried.

Is this ever likely to happen, it's already been closed and reopened once?