First, the reasons
I also installed the python2.7 and 3.5, and configure the default python to python3.5, so ros does not support, therefore suggesting no.
2. solution
By modifying the different versions of python priority, will python2.7 set to the highest, that is set as the default python.
sudo update-alternatives --install /usr/bin/python python /usr/bin/python2 200 sudo update-alternatives --install /usr/bin/python python /usr/bin/python3 150 sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.5 100