How to do it...

First, install virtualenv using pip3 and then create a new environment with the name my_flask_env inside the folder in which we ran the first command. This will create a new folder with the same name, as follows:

    $ pip3 install virtualenv
    $ virtualenv my_flask_env
  

Run the following commands from inside the my_flask_env folder:

    $ cd my_flask_env
    $ source bin/activate
    $ pip3 install flask

This will activate our environment and install Flask inside it. Now, we can do anything with our application within this environment, without affecting any other Python environment.