Pygame：让精灵以恒定的速度沿着预定路径移动

我正在Pygame从事塔防游戏，敌人会按照我为他们设定的路径前进到基地。如果您玩过VR Defender Y3K和Canyon Defense等Flash游戏，那么我的想法就是完全相同的概念。但是，我很难让精灵沿着涉及方向变化的路径（例如，路径向左或向右）移动。

所以我的问题是，让精灵沿着恒定的方向以恒定速度沿着预定路径移动的简单方法是什么？

编辑：这是我到目前为止的代码。

#Project 2 v1.00
import pygame, sys, time
from math import *
from pygame.locals import *

WHITE = (255, 255, 255)
BLACK = (0, 0, 0)
BLUE = (0, 255, 0)
RED = (255, 0, 0)
GREEN = (0, 255, 0)

surface = pygame.display.set_mode((500,500),0, 30)

class PlaceHolder:
    def __init__(self, win, left, top, width, height, color):
        self.win = win
        self.rect = pygame.Rect(left, top, width, height)
        self.color = color

    def move(self,x,y,duration):
        #self.rect = self.rect.move(self.velocity[0],self.velocity[1])
        xpointlist = [60, 60]
        ypointlist = [236, 136]
        self.rect.left += x
        self.rect.top += y
        self.duration = duration
        counter = 0
        rate = 10
        for counter in range(len(xpointlist)-1):
            dx = x - xpointlist[counter]
            dy = y - ypointlist[counter]
            distance = sqrt(dx*dx + dy*dy)
            return distance
        duration = distance/rate

    def draw(self):
        pygame.draw.ellipse(surface, self.color, self.rect, 0)

class projectWin:
    def __init__(self, width, height, caption):
        self.caption = caption
        pygame.display.set_caption(self.caption)
        self.width = width
        self.height = height
        self.placeHolders = []
        self.placeHolders.append(PlaceHolder(surface, 1, 236, 10, 10, GREEN))
        #placeHolder1 = PlaceHolder(surface, 10, 236, 10, 10, GREEN)

    def grid(self):
        for i in range(24, 499, 25): #draw the vertical lines
            pygame.draw.line(surface, WHITE, (i, 0), (i, 499), 1)
        for j in range(24, 499, 25): #draw the horizontal lines
        pygame.draw.line(surface, WHITE, (0, j), (499, j), 1)

    def path(self):
        #the two lines are separated for ease of change
        #line 1
        pygame.draw.line(surface, GREEN, (0, 224), (49, 224), 2)
        pygame.draw.line(surface, GREEN, (49, 224), (49, 149), 2)
        pygame.draw.line(surface, GREEN, (49, 149), (349, 149), 2)
        pygame.draw.line(surface, GREEN, (349, 149), (349, 274), 2)
        pygame.draw.line(surface, GREEN, (349, 274), (99, 274), 2)
        pygame.draw.line(surface, GREEN, (99, 274), (99, 349), 2)
        pygame.draw.line(surface, GREEN, (99, 349), (374, 349), 2)
        pygame.draw.line(surface, GREEN, (374, 349), (374, 224), 2)
        pygame.draw.line(surface, GREEN, (374, 224), (499, 224), 2)

        #line 2
        pygame.draw.line(surface, GREEN, (0, 249), (74, 249), 2)
        pygame.draw.line(surface, GREEN, (74, 249), (74, 174), 2)
        pygame.draw.line(surface, GREEN, (74, 174), (324, 174), 2)
        pygame.draw.line(surface, GREEN, (324, 174), (324, 249), 2)
        pygame.draw.line(surface, GREEN, (324, 249), (75, 249), 2)
        pygame.draw.line(surface, GREEN, (74, 249), (74, 374), 2)
        pygame.draw.line(surface, GREEN, (74, 374), (399, 374), 2)
        pygame.draw.line(surface, GREEN, (399, 374), (399, 249), 2)
        pygame.draw.line(surface, GREEN, (399, 249), (499, 249), 2)

    def base(self): #what the player has to protect
        base = pygame.Rect(474, 187, 99, 99)
        pygame.draw.ellipse(surface, BLUE, base, 0)

pygame.init()

win = projectWin(500, 500, 'Project 2 v1.00')
Quit = False
while not Quit:
    surface.fill(BLACK)
    #draw the grid, path, and base on the screen
    win.grid()
    win.path()
    win.base()
    for i in win.placeHolders: #draw and move the placeholder enemy assets
        i.move(59, 0, duration)
        i.move(0, -100, duration)
        i.draw()
    pygame.display.update()
    time.sleep(0.02)
    for event in pygame.event.get():
        if event.type == QUIT:
            Quit = True

pygame.quit() #always exit cleanly
sys.exit()