sift = cv2.xfeatures2d.SIFT_create() not working even though have contrib installed

I had the same problem. It seems that SIRF and SURF are no longer available in opencv > 3.4.2.16. I chose an older opencv-python and opencv-contrib-python versions and solved this problem. Here is the history version about opencv-python, and I use the following code :

pip install opencv-python==3.4.2.16
pip install opencv-contrib-python==3.4.2.16

Edit

For Anaconda User just this instead of pip

conda install -c menpo opencv

this will install cv2 3.4.1 and everything you need to run SIFT

good luck~

Edit: The opencv-contrib-python-nonfree was removed from pypi.

On Linux/ MacOS, I've found a better solution! To access nonfree detectors use: pip install opencv-contrib-python-nonfree

It may be due to a mismatch of opencv version and opencv-contrib version. If you installed opencv from the source using CMake, and the source version is different from the version of opencv-contrib-python, uninstall the current opencv-contrib-python and do pip install opencv-contrib-python==<version of the source>.X or an another compatible version. One version setup that I have running is opencv source (3.2), opencv-python (3.4.0.14) and opencv-contrib-python (3.4.2.17)