Traceback (most recent call last):
  File "hello.py", line 1, in <module>
    from flask import Flask
ImportError: No module named flask

(venv)ubuntu@localhost:/var/www/demoapp$ sudo pip install flask
Requirement already satisfied (use --upgrade to upgrade): flask in /usr/local/lib/python2.7/dist-packages
Requirement already satisfied (use --upgrade to upgrade): Werkzeug>=0.7 in /usr/local/lib/python2.7/dist-packages (from flask)
Requirement already satisfied (use --upgrade to upgrade): Jinja2>=2.4 in /usr/local/lib/python2.7/dist-packages (from flask)
Requirement already satisfied (use --upgrade to upgrade): itsdangerous>=0.21 in /usr/local/lib/python2.7/dist-packages (from flask)
Requirement already satisfied (use --upgrade to upgrade): markupsafe in /usr/local/lib/python2.7/dist-packages (from Jinja2>=2.4->flask)
Cleaning up...