import turtle
import random
pen = turtle.Turtle()
pen.color('sienna')
# 画布大小
w = turtle.Screen()
w.bgcolor('wheat')      # wheat小麦

def tree(n):
    if n >= 0 :
        if n<=12:
            pen.color(random.choice(["snow","lightcoral"]))
            pen.pensize(n/3)
        else:
            pen.pensize(n/10)
            pen.color('sienna')
        pen.speed(100)
        pen.forward(n)
        angle = random.randint(0,30)
        length = random.randint(1,15)
        pen.right( angle )
        tree(n-length)
        pen.left(2 * angle)
        tree(n-length)
        pen.penup()
        pen.right(angle)
        pen.backward(n)
        pen.down()

# 移动到起点
pen.left(90)
pen.up()
pen.backward(150)    # 后退
pen.down()
tree(60)

turtle.done()