【问题一】
背景和原因：windows python2和python3共存，环境变量只配置了Python2的，直接Pip命令调用的是Python2的。而期望是在Python3的Venv中安装。
方法：在Virtualenv中使用Pip： python -m pip install pandas
其它：直接Pycharm操作很方便；
【问题二】sudo pip时调用virtualenv外的python而不是virtual的。
(venv) [ad@CentOS bin]$ pip -V
pip 9.0.1 from /opt/flask/venv/lib/python2.7/site-packages (python 2.7)
(venv) [ad@CentOS bin]$ sudo pip -V
pip 19.0.3 from /usr/lib/python2.7/site-packages/pip (python 2.7)
(venv) [ad@CentOS bin]$ sudo /opt/flask/venv/bin/pip -V
pip 9.0.1 from /opt/flask/venv/lib/python2.7/site-packages (python 2.7)
(venv) [ad@CentOS bin]$ python -m pip -V
pip 9.0.1 from /opt/flask/venv/lib/python2.7/site-packages (python 2.7)
(venv) [ad@CentOS bin]$ sudo python -m pip -V
pip 19.0.3 from /usr/lib/python2.7/site-packages/pip (python 2.7)
(venv) [ad@CentOS /]$ sudo /opt/flask/venv/bin/python -m pip -V
pip 9.0.1 from /opt/flask/venv/lib/python2.7/site-packages (python 2.7)