Python has support for virtual environment. It’s very handy to set up isolated space where you can store specific packages for the project. You can use command virtualenv to create virtual environment.
It’s very common to add option --no-site-packages which will isolate environment from packages installed in system.
Sometimes it is necessary to revert this option.
Linux and Mac users can control it by mechanism based on one file:
venv/lib/python3.4/no-global-site-packages.txt
When this file exists Python is ignoring packages from system.
In case of Windows there is configuration file stored in venv/pyenv.cfg and you can change configuration of global packages:
Change the configuration and call Scripts\Activate.ps1 to load Python virtualenv to PowerShell.