conda update conda
conda update anaconda

conda config --add channels https://mirrors.tuna.tsinghua.edu.cn/anaconda/pkgs/free/
conda config --add channels https://mirrors.tuna.tsinghua.edu.cn/anaconda/pkgs/main/
conda config --set show_channel_urls yes 

conda env list

conda create -n python2 python=2 scikit-learn 
conda install numpy scipy sympy matplotlib -n python2

source activate python2
conda list
python -c 'import sys; print(sys.version)'
source deactivate python2

# conda install -c conda-forge jupyter_contrib_nbextensions

conda install nb_conda
conda remove nb_conda