为何Python Pygame中ball的initialvel属性始终与vel属性相等？

我用Python Pygame开发物理引擎时，遇到了一个问题：按R键重置小球的速度和位置时完全无效。测试后发现，ball.initialvel在初始化赋值后始终和ball.vel相等，每次ball.vel变化时initialvel也会跟着变，但初始化代码明明只执行了一次。

问题根源 #

pygame.Vector2是可变对象，当执行i.initialvel = i.vel这种赋值操作时，并没有复制向量的值，只是让initialvel和vel指向了内存中的同一个对象。后续对vel的任何修改（比如碰撞时反转速度、加速度累加）都会直接修改这个共享对象的数据，导致initialvel同步变化，自然无法保留初始状态。

修复方案 #

核心是给初始值创建独立的Vector2拷贝，而不是引用原对象，有两种简单的实现方式：

方式一：修改初始化循环

在给每个小球设置初始值时，使用copy()方法复制向量：

for i in ballz: 
    i.initialpos = i.pos.copy()  # 复制位置向量，生成新对象
    i.initialvel = i.vel.copy()  # 复制速度向量，生成新对象

方式二：在类的初始化方法中直接保存拷贝

把初始值的保存逻辑放到ball类的__post_init__方法里，一步到位：

def __post_init__(self):
    if self.pos is None:
        self.pos = pygame.Vector2(screen.get_width() / 2, screen.get_height() / 2)
    if self.vel is None:
        self.vel = pygame.Vector2(40,30)
    if self.accel is None:
        self.accel = g
    # 直接保存初始状态的拷贝
    self.initialpos = self.pos.copy()
    self.initialvel = self.vel.copy()

重置逻辑的优化（可选）

重置时如果希望完全隔离初始值和当前值，也可以再次拷贝初始值：

if event.type == pygame.KEYDOWN and event.key == pygame.K_r:
    for allballs in ballz:
        allballs.pos = allballs.initialpos.copy()
        allballs.vel = allballs.initialvel.copy()

完整修改后的代码 #

import numpy as np
import pygame
from dataclasses import dataclass
import random
import string

# pygame setup
pygame.init()
screen = pygame.display.set_mode((1280, 720))
clock = pygame.time.Clock()
running = True
dt = 0

#Constants and Initialisations
r = 40
M = 10
g = pygame.Vector2(0, 9.81)

@dataclass 
class ball:
    rad: float = 40
    colour: str = "red"
    vel: pygame.Vector2 = None
    pos: pygame.Vector2 = None
    accel: pygame.Vector2 = None
    mass: float = 1
    initialvel: pygame.Vector2 = None
    initialpos: pygame.Vector2 = None
    
    def __post_init__(self):
        if self.pos is None:
            self.pos = pygame.Vector2(screen.get_width() / 2, screen.get_height() / 2)
        if self.vel is None:
            self.vel = pygame.Vector2(40,30)
        if self.accel is None:
            self.accel = g
        # 保存初始状态的拷贝
        self.initialpos = self.pos.copy()
        self.initialvel = self.vel.copy()

    def BoxCollision(self):
        if self.pos.x - self.rad <= 50 or self.pos.x + self.rad >= 1230:
            self.vel.x = -self.vel.x
        if self.pos.y - self.rad <= 50 or self.pos.y + self.rad >= 670:
            self.vel.y = -self.vel.y
       
    def PosUpdate(self):
        pygame.draw.circle(screen, self.colour, self.pos, self.rad)
        self.vel += self.accel * dt
        self.BoxCollision()
        self.pos += self.vel * dt

#Declarations of balls (objects)
ballR = ball()
ballB = ball(30,"blue" , pygame.Vector2(-35,40))
ballY = ball(30,"yellow" , pygame.Vector2(-55,100))

ballz = [ballR, ballB, ballY]

#Main Loop
while running:
    for event in pygame.event.get():
        if event.type == pygame.QUIT or (event.type == pygame.KEYDOWN and event.key == pygame.K_ESCAPE):
            running = False
            
        if event.type == pygame.KEYDOWN and event.key == pygame.K_r:
            for allballs in ballz:
                print("Before reset - Initial Velocity:", allballs.initialvel)
                print("Before reset - Velocity:", allballs.vel)
                allballs.pos = allballs.initialpos.copy()
                allballs.vel = allballs.initialvel.copy()
                print("After reset - Initial Velocity:", allballs.initialvel)
                print("After reset - Velocity:", allballs.vel)
            
        if event.type == pygame.KEYDOWN and event.key == pygame.K_SPACE:
            pygame.time.wait(60000)

    screen.fill("black")
    box = pygame.draw.rect(screen, "blue", (50, 50, 1180, 620), width = 1)

    ballR.PosUpdate()
    ballB.PosUpdate()
    ballY.PosUpdate()

    pygame.display.flip()
    dt = clock.tick(144) / 100

pygame.quit()

内容的提问来源于stack exchange，提问作者rubin