2021-06-27 install ROS noetic version. There was a problem during compilation. Now record it

1. Error one

-- Could NOT find PY_em (missing: PY_EM) 
CMake Error at cmake/empy.cmake:30 (message):
Unable to find either executable 'empy' or Python module 'em'...  try
installing the package 'python-empy'

You need to install Python empy to solve the above problems

pip install empy

2. Error two

Once the error is resolved, continue with catkin_ The following error occurred when making

ImportError: "from catkin_pkg.package import parse_package" failed: No module named 'catkin_pkg'
Make sure that you have installed "catkin_pkg", it is up to date and on the PYTHONPATH

Try to find catkin_ PKG and check pythonpath with the following command:

locate catkin_pkg

If you execute the above command and report an error that locate is not installed, use the following command to install it:

sudo apt install mlocate

After executing locate catkin_ After PKG, in the displayed results, the first behavior is as follows:

/usr/lib/python3/dist-packages/catkin_pkg

View the path of pythonpath

echo $PYTHONPATH
# After executing the above command, the following result is displayed
/opt/ros/noetic/lib/python3/dist-packages

Therefore, catkin_ Make is not in the pythonpath path. Next, you need to add catkin_ Make is added to the pythonpath path

Edit the ~/.bashrc file and add the following two lines to the end of the file.
export PYTHONPATH=$PYTHONPATH:/usr/lib/python3/dist-packages

Save the file and run the source to update.
source ~/.bashrc

Re-check PYTHONPATH：
echo $PYTHONPATH
/opt/ros/noetic/lib/python3/dist-packages:/usr/lib/python3/dist-packages

To solve the above two problems