错误提示：sift = cv2.xfeatures2d.SIFT_create()
cv2.error: OpenCV(3.4.3) C:\projects\opencv-python\opencv_contrib\modules\xfeatures2d\src\sift.cpp:1207: error: (-213:The function/feature is not implemented) This algorithm is patented and is excluded in this configuration; Set OPENCV_ENABLE_NONFREE CMake option and rebuild the library in function ‘cv::xfeatures2d::SIFT::create’
解决方案：之前安装的版本太新，申请了算法专利保护，SIFT等已经不能够使用。
先卸载原先安装的库：pip uninstall opencv-python 或者 pip uninstall opencv-contrib-python，
然后pip install opencv-contrib-python==3.4.2.16 或者（3.4.2.17, 3.4.3.18, 3.4.4.19, 3.4.5.20）
我安装的是3.4.2.16，成功运行。