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How to declare an external python dependency in ros2

asked 2021-01-30 14:54:44 -0500

Loki Le Dev gravatar image

Hello, I am trying to understand what is the proper way of declaring an external python dependency in ros2-foxy.

What I want to do is create a simple python only ros2 package, and add some dependency to a python module in it, in my case it is tinymovr, that should be installed from pip.

So I create the package following this tutorial.

ros2 pkg create --build-type ament_python --node-name example_node example_python

Edit the main of the generated file to import the dependency:

import tinymovr
def main():
    print('Hi from example_python.')
if __name__ == '__main__':

Now build, source and run the package:

colcon build
source install/
ros2 run example_python example_node

Since I didn't specify anywhere the dependency, I get a nice ModuleNotFoundError

 File "~/ros2_ws/install/example_python/lib/python3.8/site-packages/example_python/", line 1, in <module>
    import tinymovr
ModuleNotFoundError: No module named 'tinymovr'

So now where should I do it?

My first try is to edit the file and edit the install_requires field:

 install_requires=['setuptools', 'tinymovr'],

But when I build and run, there is still the ModuleNotFoundError.

I read that colcon is meant only for building, and I should use rosdep to install external dependencies. And if I understand correctly I should add some <depend> tag in the package.xml file. On ubuntu it can install system packages via apt, using a package not already installed for example:


When I run

rosdep install --from-paths src --ignore-src -r -y

It nicely proposes to install it:

Continuing to install resolvable dependencies...
executing command [sudo -H apt-get install -y python3-mypy]

However I don't understand how to tell rosdep to install a package via pip. Maybe it is not supposed to do this. But why doesn't colcon install stuff from the file? I also tried with the setup.cfg file with no success.

Thank you for reading, adding examples for this case or mentioning it in the tutorial would be great.

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answered 2021-01-30 15:05:15 -0500

Loki Le Dev gravatar image

updated 2021-01-30 16:13:11 -0500

Ok after reading the rosdeptutorial I found the source for rosdep packages here:

I understand better now, also I found some explanations here:

I can either submit a change to the public rosdep/python.yaml file. Or create some local file, and then update the sources.list of rosdep in my system to point to this file.


Let's create this file : custom-python.yaml install it on a system-wide path:

sudo mkdir -p /usr/local/share/ros && sudo cp hal-python.yaml /usr/local/share/ros

Edit the rosdep sources and update:

echo "yaml file:///usr/local/share/ros/hal-python.yaml" | sudo tee -a /etc/ros/rosdep/sources.list.d/20-default.list
rosdep update

Finally install dependencies for you package:

rosdep install --from-paths src --ignore-src -r -y
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Asked: 2021-01-30 14:54:44 -0500

Seen: 357 times

Last updated: Jan 30