問題訊息如下:

$ pip install --upgrade pip
Traceback (most recent call last):
  File "/usr/local/bin/pip", line 6, in <module>
    from pip._internal.cli.main import main
ModuleNotFoundError: No module named 'pip._internal.cli.main'

解決方式:

curl https://bootstrap.pypa.io/get-pip.py -o get-pip.py
python3 get-pip.py --force-reinstall

Defaulting to user installation because normal site-packages is not writeable
Collecting pip
  Using cached pip-21.3.1-py3-none-any.whl (1.7 MB)
Installing collected packages: pip
  WARNING: The scripts pip, pip3 and pip3.8 are installed in '/Users/bocky/Library/Python/3.8/bin' which is not on PATH.
  Consider adding this directory to PATH or, if you prefer to suppress this warning, use --no-warn-script-location.
Successfully installed pip-21.3.1
WARNING: You are using pip version 19.2.3; however, version 21.3.1 is available.
You should consider upgrading via the '/Applications/Xcode.app/Contents/Developer/usr/bin/python3 -m pip install --upgrade pip' command.

訊息告知建議執行的內容:

/Applications/Xcode.app/Contents/Developer/usr/bin/python3 -m pip install --upgrade pip

下方為執行結果:

Defaulting to user installation because normal site-packages is not writeable
Requirement already satisfied: pip in ./Library/Python/3.8/lib/python/site-packages (21.3.1)

解決!