After upgrading your system Python version, you may experience that your old venvs (virtual environments) no longer work. In this case, you need to create a new venv and install the same packages as in the old venv. However, you cannot run pip freeze in the old venv (because you uninstalled the version of Python used by that venv).

This script does the equivalent of the following:

• venv/bin/python3.6 -m pip freeze > /tmp/requirements.txt
• mv venv venv~
• python3.7 -m venv venv
• venv/bin/python3.7 -m pip install -r /tmp/requirements.txt

(however, without invoking the old python3.6 and without creating /tmp/requirements.txt.)