4.5. Flow of Execution of the for Loop

range(4) tương đương với 0, 1, 2, 3
range (1,5) ==> 1, 2, 3, 4

• Turtle methods can use negative angles or distances. So tess.forward(-100) will move tess backwards, and tess.left(-30) turns her to the right. Additionally, because there are 360 degrees in a circle, turning 30 to the left will leave you facing in the same direction as turning 330 to the right! (The on-screen animation will differ, though — you will be able to tell if tess is turning clockwise or counter-clockwise!)
  This suggests that we don’t need both a left and a right turn method — we could be minimalists, and just have one method. There is also a backward method. (If you are very nerdy, you might enjoy saying alex.backward(-100) to move alex forward!)
  Part of thinking like a scientist is to understand more of the structure and rich relationships in your field. So reviewing a few basic facts about geometry and number lines, like we’ve done here is a good start if we’re going to play with turtles.
• A turtle’s pen can be picked up or put down. This allows us to move a turtle to a different place without drawing a line. The methods are up and down. Note that the methods penup and pendowndo the same thing.

  alex.up()
  alex.forward(100)     # this moves alex, but no line is drawn
  alex.down()
  

• Every turtle can have its own shape. The ones available “out of the box” are arrow, blank, circle, classic, square, triangle, turtle.

  ...
  alex.shape("turtle")
  ...
  

• You can speed up or slow down the turtle’s animation speed. (Animation controls how quickly the turtle turns and moves forward). Speed settings can be set between 1 (slowest) to 10 (fastest). But if you set the speed to 0, it has a special meaning — turn off animation and go as fast as possible.

  alex.speed(10)
  

• A turtle can “stamp” its footprint onto the canvas, and this will remain after the turtle has moved somewhere else. Stamping works even when the pen is up.

import turtle

wn = turtle.Screen()

wn.bgcolor("lightgreen")

tess = turtle.Turtle()

tess.color("blue")

tess.shape("turtle")

print(list(range(5, 60, 2)))

tess.up()                     # this is new

for size in range(5, 60, 2):    # start with size = 5 and grow by 2

    tess.stamp()                # leave an impression on the canvas

    tess.forward(size)          # move tess along

    tess.right(24)              # and turn her

wn.exitonclick()