$PYTHONPATH is not in sync with Python sys.path

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I have build ROS Melodic on a Raspberry Pi using the supplied instructions. This seems to work fine and I'm able to build my own workspace.

However, after sourcing setup.bash, the $PYTHONPATH variable is not in line with what Python reports as its path. Thus certain libraries can not be found when executing ROS programs (eg std_msgs needs PyYAML, which is installed, but not available when running the project through roslaunch)

Is this a misconfiguration on my end? What can I verify and change to make this work?

pi@raspberrypi:~ $ echo $PYTHONPATH
/home/pi/ros-robot-control/HelloWorld/robot_ws/devel/lib/python2.7/dist-packages:/opt/ros/melodic/lib/python2.7/dist-packages
pi@raspberrypi:~ $ python
Python 2.7.16 (default, Oct 10 2019, 22:02:15)
[GCC 8.3.0] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> print(sys.path)
['', '/home/pi/ros-robot-control/HelloWorld/robot_ws/devel/lib/python2.7/dist-packages', '/opt/ros/melodic/lib/python2.7/dist-packages', '/usr/lib/python2.7', '/usr/lib/python2.7/plat-arm-linux-gnueabihf', '/usr/lib/python2.7/lib-tk', '/usr/lib/python2.7/lib-old', '/usr/lib/python2.7/lib-dynload', '/home/pi/.local/lib/python2.7/site-packages', '/usr/local/lib/python2.7/dist-packages', '/usr/lib/python2.7/dist-packages', '/usr/lib/python2.7/dist-packages/gtk-2.0', '/usr/lib/python2.7/dist-packages/wx-3.0-gtk3']

The error when using roslaunch:

Traceback (most recent call last):
  File "/home/pi/ros-robot-control/HelloWorld/robot_ws/src/ros_robot_control/scripts/cmd_vel_listener.py", line 2, in <module>
    import rospy
  File "/opt/ros/melodic/lib/python2.7/dist-packages/rospy/__init__.py", line 47, in <module>
    from std_msgs.msg import Header
  File "/opt/ros/melodic/lib/python2.7/dist-packages/std_msgs/msg/__init__.py", line 1, in <module>
    from ._Bool import *
  File "/opt/ros/melodic/lib/python2.7/dist-packages/std_msgs/msg/_Bool.py", line 5, in <module>
    import genpy
  File "/opt/ros/melodic/lib/python2.7/dist-packages/genpy/__init__.py", line 34, in <module>
    from . message import Message, SerializationError, DeserializationError, MessageException, struct_I
  File "/opt/ros/melodic/lib/python2.7/dist-packages/genpy/message.py", line 47, in <module>
    import yaml
ModuleNotFoundError: No module named 'yaml'

pyyaml is installed, but not in any of the folders in $PYTHONPATH:

pi@raspberrypi:~/ros-robot-control/HelloWorld/robot_ws/src/robot_interface $ pip install pyyaml
Looking in indexes: https://pypi.org/simple, https://www.piwheels.org/simple
Requirement already satisfied: pyyaml in /usr/lib/python2.7/dist-packages (3.13)