import turtle

pen = turtle.Turtle()
pen.color('white')
# 画布大小
w = turtle.Screen()
w.bgcolor('black')      # 小麦
pen.pensize(5)
# 移动到起点
pen.left(90)
pen.up()
pen.backward(150)    # 前进4
pen.down()


def tree(n):
    if n >=1:
        pen.forward(n)
        pen.right(30)
        tree(n-10)
        pen.left(60)
        tree(n-10)
        pen.right(30)
        pen.up
        pen.backward(n)
        pen.down()
tree(50)
turtle.done()