Python imports.md

Python imports

Welcome (back) to the "but it worked so well with ROS1!" camp. Here you will find some hair-pulling avoidance tips for python imports. These are functional simplifications that'll get you going without reading language and ros specs.

What? If you import like from myfile import mything with ROS2, this file won't be able to import when you colcon build and launch the node. If you import like from .myfile import mything (notice the dot) then it will run fine with a launch, but now you can not run the file with just python3 file.py.

Why?! It worked in ROS1 because of a hack that catkin allowed. Colcon does not allow the same hack, thus it doesn't work the same way with ROS2.

This is consistent with the general theme of bringing C++ and Python closer together within the context of ROS2. Like with C++, you now need to build and use launch files to test python stuff.

Dirty workaround

It wouldn't be python without a funny workaround:

try:
    from myfile import mything # for terminal
except:
    from .myfile import mything # for ROS2

Slightly less dirty

If some imports are ONLY used for running from the terminal, you can also put them under the main: if __name__ == "__main__":, these imports will only be ran when you run the file by itself and not when ROS runs it.

How about folders?

If you want something like this:

colcon_ws
|- yourpackage
|  |- yourpackage
|  |- __init__.py
|  |- main.py
|  |- module_folder
|  |  |- __init__.py
|  |  |- module1.py
|  |  |- module2.py

And you want to import like from module_folder.module1 import func in main.py then you need to tell colcon that module_folder is in fact a module folder.

1. Notice the __init__.py in module_folder.
2. Add that folder to your setup.py like so: package = [yourpackage, yourpackage/module_folder] (this line should be there if you used ros2 pkg to create your package)
3. Import as from .module_folder.module1 import thing (period needed for ROS2, shouldn't be there for non-ros2)