``` 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) ws 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() ``` ``` from turtle import * from random import * from math import * def tree(n,l): pd()#下笔 #阴影效果 t = cos(radians(heading()+45))/8+0.25 pencolor(t,t,t) pensize(n/3) forward(l)#画树枝 if n>0: b = random()*15+10 #右分支偏转角度 c = random()*15+10 #左分支偏转角度 d = l*(random()*0.25+0.7) #下一个分支的长度 #右转一定角度,画右分支 right(b) tree(n-1,d) #左转一定角度，画左分支 left(b+c) tree(n-1,d) #转回来 right(c) else: #画叶子 right(90) n=cos(radians(heading()-45))/4+0.5 pencolor(n,n*0.8,n*0.8) circle(3) left(90) #添加0.3倍的飘落叶子 if(random()>0.7): pu() #飘落 t = heading() an = -40 +random()*40 setheading(an) dis = int(800*random()*0.5 + 400*random()*0.3 + 200*random()*0.2) forward(dis) setheading(t) #画叶子 pd() right(90) n = cos(radians(heading()-45))/4+0.5 pencolor(n*0.5+0.5,0.4+n*0.4,0.4+n*0.4) circle(2) left(90) pu() #返回 t=heading() setheading(an) backward(dis) setheading(t) pu() backward(l)#退回 bgcolor(0.5,0.5,0.5)#背景色 ht()#隐藏turtle speed(0)#速度 1-10渐进，0 最快 tracer(0,0) pu()#抬笔 backward(100) left(90)#左转90度 pu()#抬笔 backward(300)#后退300 tree(12,100)#递归7层 done() ``` ``` >>> letters ['a', 'b', 'c', 'd', 'e', 'f', 'g'] >>> letters[2:5] ['c', 'd', 'e'] >>> letters[2:5] = ['C', 'D', 'E'] >>> letters ['a', 'b', 'C', 'D', 'E', 'f', 'g'] >>> letters[:5] ['a', 'b', 'C', 'D', 'E'] >>> letters[0:5] ['a', 'b', 'C', 'D', 'E'] >>> letters[5:] ['f', 'g'] >>> letters[-2:] ['f', 'g'] >>> letters[-2:0] [] >>> letters[-2:-1] ['f'] >>> letters[-2:] ['f', 'g'] >>> letters[-2:] ['f', 'g'] >>> letters[-2:-1] ['f'] >>> letters[-2:] ['f', 'g'] >>> letters[-2:-3] [] >>> letters[-3:-2] ['E'] >>> letters[-2:-3:-1] ['f'] >>> letters[-2:-3:1] [] >>> letters[-2:-3:] [] >>> letters[-2:-3] [] >>> letters[-2:-3:-1] ['f'] >>> letters[2:-2] ['C', 'D', 'E'] >>> letters ['a', 'b', 'C', 'D', 'E', 'f', 'g'] >>> letters[2:-2] ['C', 'D', 'E'] >>> letters[2:-2] ['C', 'D', 'E'] >>> letters[2:-2:1] ['C', 'D', 'E'] >>> letters[2:-2:2] ['C', 'E'] >>> letters[2:-2:-2] [] >>> letters[-2:2:-2] ['f', 'D'] ```