Got "/opt/stack/requirements/.venv/bin/pip’: No such file or directory" error installing devstack

Strangely tonight when trying to install devstack for a new experiment, after a couple failing attempts (mostly with directory permissions), I got this message saying:

"/opt/stack/requirements/.venv/bin/pip’: No such file or directory"

What happens?

Turns out it is just virtualenv was failed to install pip in the previous try. Fortunately, I found an easy fix:

$ virtualenv /opt/stack/requirements/.venv/

Then I just need to execute again.