# drawtree.py
 
from turtle import Turtle, mainloop
 
def tree(plist, l, a, f):
  """ plist is list of pens
  l is length of branch
  a is half of the angle between 2 branches
  f is factor by which branch is shortened
  from level to level."""
  if l > 5: #
    lst = []
    for p in plist:
      p.forward(l)#沿着当前的方向画画Move the turtle forward by the specified distance, in the direction the turtle is headed.
      q = p.clone()#Create and return a clone of the turtle with same position, heading and turtle properties.
      p.left(a) #Turn turtle left by angle units
      q.right(a)# turn turtle right by angle units, nits are by default degrees, but can be set via the degrees() and radians() functions.
      lst.append(p)#将元素增加到列表的最后
      lst.append(q)
    tree(lst, l*f, a, f)
  
      
 
def main():
  p = Turtle()
  p.color("green")
  p.pensize(5)
  #p.setundobuffer(None)
  p.hideturtle() #Make the turtle invisible. It's a good idea to do this while you're in the middle of doing some complex drawing,
  #because hiding the turtle speeds up the drawing observably.
  #p.speed(10)
  # p.getscreen().tracer(1,0)#Return the TurtleScreen object the turtle is drawing on.
  p.speed(10)
  #TurtleScreen methods can then be called for that object.
  p.left(90)# Turn turtle left by angle units. direction 调整画笔
 
  p.penup() #Pull the pen up – no drawing when moving.
  p.goto(0,-200)#Move turtle to an absolute position. If the pen is down, draw line. Do not change the turtle's orientation.
  p.pendown()# Pull the pen down – drawing when moving. 这三条语句是一个组合相当于先把笔收起来再移动到指定位置，再把笔放下开始画
  #否则turtle一移动就会自动的把线画出来
 
  #t = tree([p], 200, 65, 0.6375)
  t = tree([p], 200, 65, 0.6375)
   
main()