python sprites随机移动使聚类保持在左上角

Question

我遇到的问题似乎在random_movement函数中。它会继续在170左右吐出相同的数字，这会使子画面在屏幕的左上角聚集。任何想法为何以及我可能会做些什么？

我想要精灵执行的操作是移动一小段距离然后改变方向并再移动一点。

def avoid_mobs(self):
    for mob in self.game.mobs:
        if mob != self:
            dist = self.pos - mob.pos
            if 0 < dist.length() < AVOID_RADIUS:
                self.acc += dist.normalize()



def random_movement(self):
    new_angle = uniform(-1, 1)
    new_x = math.sin(new_angle * math.pi/2)
    new_y = math.cos(new_angle * math.pi/2)
    new_vector = pg.math.Vector2(new_x,new_y)
    new_vector.normalize()
    new_vector *= randrange(360)            
    print ('new_vec: ' + str(new_vector))
    return new_vector

def chase(self, target_dist):
    self.rot = target_dist.angle_to(vec(1, 0))                
    print ('rot: ' + str(self.rot))
    print ('vel: ' + str(self.vel))
    self.image = pg.transform.rotate(self.game.mob_img, self.rot)
    self.rect = self.image.get_rect()
    self.rect.center = self.pos
    self.acc = vec(1, 0).rotate(-self.rot)
    self.avoid_mobs()
    self.acc.scale_to_length(self.speed)
    self.acc += self.vel * -1
    self.vel += self.acc * self.game.dt
    self.pos += self.vel * self.game.dt + 0.5 * self.acc * self.game.dt ** 2
    self.hit_rect.centerx = self.pos.x
    collide_with_walls(self, self.game.walls, 'x')
    self.hit_rect.centery = self.pos.y
    collide_with_walls(self, self.game.walls, 'y')
    self.rect.center = self.hit_rect.center


def update(self):
    #target_dist = self.target.pos - self.pos
    #if target_dist.length_squared() < DETECT_RADIUS ** 2:
    #    # Play zombie moans once in a while
    #    if random() < 0.002:    
    #        choice(self.game.zombie_moan_sounds).play()
    #    self.wander = target_dist
    ##    self.chase(target_dist)
    #else:
    if random() < 0.05:
        self.wander = self.random_movement() - self.pos
        print ('target' + str(self.wander))
    self.chase(self.wander)
    if self.health <= 0:
        snd = self.game.zombie_death_sound[0]
        if snd.get_num_channels() > 2:
            snd.stop()
        snd.play()
        self.kill()
        self.game.map_img.blit(self.game.splat, self.pos - vec(32, 32))

def draw_health(self):
    if self.health > 60:
        col = GREEN
    elif self.health > 30:
        col = YELLOW
    else:
        col = RED
    width = int(self.rect.width * self.health / 100)
    self.health_bar = pg.Rect(0, 0, width, 7)
    if self.health < 100:
        pg.draw.rect(self.image, col, self.health_bar)