参考：https://blog.csdn.net/u013468614/article/details/115460030
在虚拟环境中安装cv2，报错如下：

pip install opencv-python
Exception:
Traceback (most recent call last):
  File "/home/zl/anaconda3/envs/tools/lib/python3.5/site-packages/pip/_internal/basecommand.py", line 228, in main
    status = self.run(options, args)
  File "/home/zl/anaconda3/envs/tools/lib/python3.5/site-packages/pip/_internal/commands/install.py", line 241, in run
    with self._build_session(options) as session:
  File "/home/zl/anaconda3/envs/tools/lib/python3.5/site-packages/pip/_internal/basecommand.py", line 78, in _build_session
    if options.cache_dir else None
  File "/home/zl/anaconda3/envs/tools/lib/python3.5/posixpath.py", line 89, in join
    genericpath._check_arg_types('join', a, *p)
  File "/home/zl/anaconda3/envs/tools/lib/python3.5/genericpath.py", line 143, in _check_arg_types
    (funcname, s.__class__.__name__)) from None
TypeError: join() argument must be str or bytes, not 'int'

当偿试用pip install --upgrade pip升级pip出现上面一样的错误。这样就陷入了死循环。
解决

又是报复性搜索，终于…

点开Ubuntu 16.04安装最新的pip给出的官方的安装指引文件安装方式

curl https://bootstrap.pypa.io/get-pip.py -o get-pip.py
python get-pip.py

python get-pip.py
ERROR: This script does not work on Python 3.5 The minimum supported Python version is 3.7. Please use https://bootstrap.pypa.io/pip/3.5/get-pip.py instead.

换成建议的地址试试：

curl https://bootstrap.pypa.io/pip/3.5/get-pip.py -o get-pip.py
python get-pip.py

测试，安装成功：

pip -V
pip 20.3.4 from /home/zl/anaconda3/envs/tools/lib/python3.5/site-packages/pip (python 3.5)