Dave met this problem when trying to recognize traffic light. Importing python function failed. To solve the problem，try install opencv-contrib-python. Someone said that aruco is not in opencv-python, but in opencv-contrib-python.

Solutions

For python2

pip install --upgrade pip
pip install opencv-contrib-python

For python3

pip3 install --upgrade pip
pip3 install opencv-contrib-python

Reference

Python opencv Aruco “No module named ‘cv2.aruco’”

How to upgrade pip3?