ImportError while installing ROS2 from source

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I followed the instructions for installing ROS2 from source (from github wiki).

Isolated and non-isolated build:

src/ament/ament_tools/scripts/ament.py build --build-tests --symlink-install <--isolated>: I'm getting a Import Error: No module named 'catkin_pkg' and a NameError: name 'catkin_pkg' is not defined. The errors occur at line number 34 and 48 in ros1_bridge/__init__.py.

$ <path-to-python> -c 'import catkin_pkg' reproduces this error.

Any ideas why? Is there a step that I've missed? Should there be a sudo -H pip install catkin_pkg rospkg somewhere in the docs?