The first time around, run:

$ virtualenv venv
$ source venv/bin/activate
$ pip install flask flask-jsonpify flask-sqlalchemy flask-restful requests
$ pip freeze

This creates a new virtual environment, installs necessary dependencies and fixes those to the custom created environment called venv.

From the second time onward, run:

$ source venv/bin/activate

This reactivates a previously created virtual env, so that all installed dependencies are in place.

To deactivate the virtual environment, simply execute deactivate.

$ python server.py

• Building a basic RESTful API with Python
• How to use Python virtualenv