蔡佩臻PYTHON條件判斷if否則elif

from tkinter import * #或者import tkinter as tk
import math
tk = Tk()           #建構視窗名為tk
tk.geometry('1200x400')#視窗 寬1200像素
tk.title("蔡佩臻python tkinter輸入數學函式庫")
canvas = Canvas(tk, width=1200, height=400, bg='gainsboro')
canvas.grid(row=0,column=0,padx=5,pady=5,columnspan=3)
delay=1 # milliseconds, 1/1000秒
x1,y1,z1=0,200,10#python特徵,多變數=對等值
h=190      #上下範圍,相當於數學1到-1
def LH():
    global x1, y1, z1,inc#global全球,local當地 
    if(x1 == 0):
        inc = 1
        canvas.delete("all") 
    elif(x1==1200):
        inc=-1
        canvas.delete("all")
    x2 = x1 + inc
    y2=200 - h*math.sin(0.02*x2)
    z2=200 - h*math.cos(0.02*x2) 
    L1=canvas.create_line(x1,y1,x2,y2,fill='lemon chiffon',width=10)
    L2=canvas.create_line(x1,z1,x2,z2,fill='linen',width=10)   
    x1,y1,z1=x2,y2,z2#下一個起點是現在終點
    canvas.after(delay,LH)
LH()            #執行LauHou老猴
tk.mainloop()