So far we have accomplished predefined tasks, but in all honesty we were accomplishing no better achievements than the 18th century automata, or the music boxes following one set of instruction to the end. What makes programming so much more powerful are conditional statements. This is the ability to test a variable against a value and act in one way if the condition is met by the variable or an other way if not. They are also commonly called by programmers if statements.

To know if a condition is True of False, we need a new type of data: the booleans. They allow logical operations. A logic statement or operation can be evaluated to be True or False. Our conditional statement can then be understood like this:

if (evaluation of a condition returns true) then do some operation else do an other operation. And any operation that can be evaluated as True or False can be put to the test. All comparisons return True or False: ==, >, <.

Here are some simple examples:

condition = True
if condition:
    print("condition met")

if not condition:
    print("condition not met")

direction = -30
if direction > 0 :
    turtle.forward(direction)
else:
    turtle.left(180)
    turtle.forward(-direction)

Let's try to make our turtle stop when it reaches the end of the window. Let's only take into account horizontal movements. We want the turtle to go forward for a specified distance (taken as a function parameter) until it reaches this distance and stops.

def stop_at_end_of_screen(distance):
    future_x_coord = turtle.xcor() + distance
    diff = turtle.window_width()/2 - future_x_coord
    if  diff > 0 :
        turtle.forward(distance)
    else:
        turtle.forward(turtle.window_width()/2 - turtle.xcor())

Now when the turtle reaches the end of the screen, it turns around and continue its movement in the other direction, the remaining of the distance

def uturn_at_border(distance):
    if turtle.heading() == 0 :
        future_x_coord = turtle.xcor() + distance
    else:
        future_x_coord = turtle.xcor() - distance
    diff = turtle.window_width()/2 - future_x_coord
    if  diff > 0 :
        turtle.forward(distance)
    else:
        turtle.forward(turtle.window_width()/2 - turtle.xcor())
        turtle.left(180)
        turtle.forward(-diff)