电脑上同时有python2 和python3，python2 里有cv2，Python3里没有。

用pip3 install opencv-python 安装，提示：

You are using pip version 9.0.1, however version 10.0.1 is available.
You should consider upgrading via the 'pip install --upgrade pip' command.

所以先升级pip3,但是提示这个命令没用，使用命令：python3 -m pip install --upgrade pip，提示：

Successfully uninstalled pip-9.0.1
Successfully installed pip-10.0.1

再用pip3 install opencv-python，然后就可以在Python3 里import cv2 了。