The turtle module using screen coordinates such that the vertical coordinate increases vertically on the screen. This orientation is different from the traditional orientation for graphics programs.

It is possible to set coordinates with the "normal" orientation using the setworldcoordinates function, e.g.

    >>> from turtle import *
    >>> setworldcoordinates(0, 400, 400, 0)

With the above choice, the y coordinate increases as the turtle moves down on the screen.  However, doing so inverts the role of left() and right().

I have attached a "diff" file which introduces a change needed so that left() and right() behave correctly with this different orientation.