经历
python3执行python脚本import cv2报错，发现原因是指定的python2.7目录下的cv2，后来为了在安装python3版本的opencv，安装了pip3,结果安装opencv报错，然后升级了pip3,然后pip的所有指令都不能用了。。。一直报错

Traceback (most recent call last):
  File "/usr/bin/pip3", line 11, in <module>
    sys.exit(main())
  File "/home/qin/.local/lib/python3.5/site-packages/pip/__init__.py", line 12, in main
    from pip._internal.utils.entrypoints import _wrapper
  File "/home/qin/.local/lib/python3.5/site-packages/pip/_internal/utils/entrypoints.py", line 4, in <module>
    from pip._internal.cli.main import main
  File "/home/qin/.local/lib/python3.5/site-packages/pip/_internal/cli/main.py", line 58
    sys.stderr.write(f"ERROR: {exc}")
                                   ^
SyntaxError: invalid syntax

原因
查了很多，大多数老外的说法是，高版本的pip不再支持python3.5

解决

1. 卸载pip
   sudo apt remove --purge python3-pip

2. 移除pip的相关文件
   sudo apt autoremove

3. 下载pip的安装文件
   curl -O https://bootstrap.pypa.io/pip/3.5/get-pip.py

4. 执行安装文件
   /usr/bin/python3 /home/qin/下载/rosbag2video-master/get-pip.py

5. pip降级
   sudo -E python3 -m pip install --upgrade "pip < 19.2"

大功告成，希望对大家有所帮助