防禦式編程的重點就是需要防禦一些程式未曾預料的錯誤，這是一種提高軟體質量的輔助性方法，斷言assert就用於防禦式編程，編寫代碼時，我們總是會做出一些假設，斷言就是用於在代碼中捕捉這些假設。使用斷言是為了驗證預期的結果——當程式執行到斷言的位置時，對應的斷言應該為真；若斷言不為真時，程式會終止執行， ...
更換方法
Ubuntu採用apt作為軟體安裝工具，其鏡像源列表記錄在/etc/apt/source.list文件中。
首先將source.list複製為source.list.bak備份，然後將source.list內容改為需要的鏡像源列表即可。
修改完成後保存source.list文件，執行：
sudo apt update
等待更新完成即可。
常用國內鏡像源
本節均為 Ubuntu 20.04 的鏡像源列表。若為其他版本，將所有focal更改為其他版本代號即可。
常用的Ubuntu版本代號如下：
- Ubuntu 22.04：jammy
- Ubuntu 20.04：focal
- Ubuntu 18.04：bionic
- Ubuntu 16.04：xenial
Ubuntu 通常採用“形容詞+小動物”作為版本代號（預設的壁紙），在鏡像源列表中只有第一個詞。
此外，預設註釋了代碼源以提高速度，註釋了預發佈軟體源（可能不穩定）。如有需要可以取消註釋。
建議將所有常用鏡像源保存在/etc/apt目錄下，並命名為類似source.list.aliyun的形式，需要使用時直接複製替換source.list文件即可。
阿裡雲
deb http://mirrors.aliyun.com/ubuntu/ focal main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-security main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-updates main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-backports main restricted universe multiverse
# deb-src http://mirrors.aliyun.com/ubuntu/ focal main restricted universe multiverse
# deb-src http://mirrors.aliyun.com/ubuntu/ focal-security main restricted universe multiverse
# deb-src http://mirrors.aliyun.com/ubuntu/ focal-updates main restricted universe multiverse
# deb-src http://mirrors.aliyun.com/ubuntu/ focal-backports main restricted universe multiverse
## Pre-released source, not recommended.
# deb http://mirrors.aliyun.com/ubuntu/ focal-proposed main restricted universe multiverse
# deb-src http://mirrors.aliyun.com/ubuntu/ focal-proposed main restricted universe multiverse
清華
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-updates main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-backports main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-security main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-updates main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-backports main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-security main restricted universe multiverse
## Pre-released source, not recommended.
# deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-proposed main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-proposed main restricted universe multiverse
中科大
deb https://mirrors.ustc.edu.cn/ubuntu/ focal main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ focal-updates main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ focal-backports main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ focal-security main restricted universe multiverse
# deb-src https://mirrors.ustc.edu.cn/ubuntu/ focal main restricted universe multiverse
# deb-src https://mirrors.ustc.edu.cn/ubuntu/ focal-updates main restricted universe multiverse
# deb-src https://mirrors.ustc.edu.cn/ubuntu/ focal-backports main restricted universe multiverse
# deb-src https://mirrors.ustc.edu.cn/ubuntu/ focal-security main restricted universe multiverse
## Pre-released source, not recommended.
# deb https://mirrors.ustc.edu.cn/ubuntu/ focal-proposed main restricted universe multiverse
# deb-src https://mirrors.ustc.edu.cn/ubuntu/ focal-proposed main restricted universe multiverse
網易163
deb http://mirrors.163.com/ubuntu/ focal main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ focal-security main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ focal-updates main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ focal-backports main restricted universe multiverse
# deb-src http://mirrors.163.com/ubuntu/ focal main restricted universe multiverse
# deb-src http://mirrors.163.com/ubuntu/ focal-security main restricted universe multiverse
# deb-src http://mirrors.163.com/ubuntu/ focal-updates main restricted universe multiverse
# deb-src http://mirrors.163.com/ubuntu/ focal-backports main restricted universe multiverse
## Pre-released source, not recommended.
# deb http://mirrors.163.com/ubuntu/ focal-proposed main restricted universe multiverse
# deb-src http://mirrors.163.com/ubuntu/ focal-proposed main restricted universe multiverse
