PIP error: externally-managed-environment ( How to use virtual environment python)

 I encountered an error "error: externally-managed-environment" when trying to install library in Ubuntu. The message said to use "virtual environment" or risk breaking system. 

To create a virtual environment and install 

python -m venv my-venv

my-venv/bin/pip install some-python-library

This creates folder my-venv in the current location. 

To use virtual environment, activate 

source myenv/bin/activate

This enters virtual environment. 

deactivate