Installed Flask in a virtualenv yet "command not found"

I ran into this issue while running through the tutorial for version 0.12, so for people who find this thread hitting this issue with a later version, I was able to get the server running by using:

$ python -m flask run

Flask 0.10 has no flask command, it was added in 0.11. If pi.py has the smarts to run your app, such as if it's using Flask-Script, the command you're looking for is:

$ python pi.py

You can install Flask-CLI to get the flask command in 0.10 if you can't upgrade to 0.11.