Logic error in python turtle

Question

I am coding in python 3.2 turtle and I have this beautiful drawing of a tank. and I know how to move it left and write. However, when trying to make the tank move up and dow

Answer 1

When drawing a custom cursor, unless you have a reason not to do so, keep your cursor design centered about the origin, (0, 0). If you draw to one side of the origin, then when your turtle turns right or left, it won't do so symmetrically. That is, as it turns in one direction, it will appear to do so quickly but in the opposite direction, it will appear to "take the long way around".

I've worked the polygons underlying the tank cursor so that it's center is the center of the turret and removed the setheading() calls from the drawing routines.

This example uses what I think is consistent motion: regardless of tank orientation, 'up' is forward, 'down' is backward, 'left' is 90 degrees to the left and 'right' is 90 degrees to the right. I've also stripped down the example just to demonstrate the motion:

import turtle

# Defining shapes
def polySquare(t, x, y, length):
    t.goto(x, y)
    t.begin_poly()
    for count in range(4):
        t.forward(length)
        t.left(90)
    t.end_poly()
    return t.get_poly()

def polyRectangle(t, x, y, length1, length2):
    t.goto(x, y)
    t.begin_poly()
    for count in range(2):
        t.forward(length1)
        t.left(90)
        t.forward(length2)
        t.left(90)
    t.end_poly()
    return t.get_poly()

def tankCursor():
    """
    Create the tank cursor.
    """
    temporary = turtle.Turtle()
    temporary.hideturtle()
    temporary.penup()

    screen = turtle.getscreen()
    delay = screen.delay()
    screen.delay(0)

    tank = turtle.Shape("compound")
    tire1 = polyRectangle(temporary, -65, -75, 30, 75)  # Tire #1
    tank.addcomponent(tire1, "gray", "black")
    tire2 = polyRectangle(temporary, 35, -75, 30, 75)  # Tire #2
    tank.addcomponent(tire2, "gray", "black")
    tire3 = polyRectangle(temporary, 35, 0, 30, 75)  # Tire #3
    tank.addcomponent(tire3, "gray", "black")
    tire4 = polyRectangle(temporary, -65, 0, 30, 75)  # Tire #4
    tank.addcomponent(tire4, "gray", "black")
    bodyTank = polyRectangle(temporary, -55, -65, 110, 130)
    tank.addcomponent(bodyTank, "black", "gray")
    gunTank = polyRectangle(temporary, -10, 25, 20, 100)   # Gun
    tank.addcomponent(gunTank, "black", "gray")
    exhaustTank = polyRectangle(temporary, -25, -75, 10, 20)
    tank.addcomponent(exhaustTank, "black", "gray")
    turretTank = polySquare(temporary, -25, -25, 50)  # Turret
    tank.addcomponent(turretTank, "red", "gray")
    turtle.addshape("tank", shape=tank)

    del temporary
    screen.delay(delay)

tankCursor()  # creates and registers the "tank" cursor shape
turtle.shape("tank")
turtle.up()  # get rid of the ink

# Movement of tank
screen = turtle.Screen()
screen.onkeypress(lambda : turtle.right(90), "Right")
screen.onkeypress(lambda : turtle.left(90), "Left")
screen.onkeypress(lambda : turtle.forward(40), "Up")
screen.onkeypress(lambda : turtle.backward(40), "Down")

turtle.listen()

turtle.mainloop()