1) install
sudo apt install -i https://pypi.douban.com/simple python3-venv virtualenv python3-virtualenv virtualenvwrapper

2) setting
vi ~/.bashrc
===========
export WORKON_HOME=$HOME/.virtualenvs
source /usr/local/bin/virtualenvwrapper.sh

export PYTHONPATH=/opt/shared/library/mantid-4.2.3/lib:$PYTHONPATH
export PATH=/opt/shared/library/mantid-4.2.3/bin:$PATH
export PYTHONPATH=/opt/shared/library/mantid-4.2.3/bin:$PYTHONPATH
===========
source ~/.bashrc

3) create env
mkvirtualenv -p python3 flask

4) swith env
workon flask

5) install module
pip install -i https://pypi.douban.com/simple -r requirements.txt
pip freeze >requirements.txt

6) deactivate env
deactivate

7) remove env
rmvirtualenv flask
