Including a Python module in a ROS2 package

Question

Hey you all I have been scratching at my head for a solid day trying to figure out why this is proving to be such a problem.

So i have a python ROs2 package. Inside my package i have a node trying to reference a submodule i have included. It keeps saying there is no module in package. I have tried to look at the setup.py file and still have had no luck trying to add it as part of the path.

Project Folder

    A [package] 

         A 

            nodeFile.py 

            Submodule 

                 file-that-i-am-trying-to-call.py

.....

Sos. I havent added anything unique to the setup.py or the package.xml from a typical basic node setup so its pretty blank in terms of that.

Any help would be awesome thank you so much!!

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Originally posted by edesta on ROS Answers with karma: 51 on 2020-12-15

Post score: 5

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Original comments

Comment by nnmm on 2020-12-16:
What is the path you use for importing that submodule? Do you have an init.py?

Comment by edesta on 2020-12-16:
to import it i said from Submodule.file_that-i-am-trying-to-call.py and yes it has an init.py

An interesting thing that happens is when i remove the submodule folder and pull the file in by itself, i can import it like: .file-that-i-am-trying-to-call.py

Comment by nnmm on 2020-12-16:
You shouldn't use dashes in module names, and you shouldn't need the .py extension when importing. I think theoretically it should be import A.Submodule.file_that_i_am_trying_to_call (i.e. with the package name in front). If that doesn't work, please provide a minimal example to reproduce the error.

Comment by edesta on 2020-12-16:
Hi, So this is for the example i used dashes for the actual implementation i will give you the set of steps to recreate.

I made a python ros2 package just using the customary build instructions I then made a node that includes a submodule for a dstar algorithm implementation.

so the folder (called dstar) contains dstar_algorithm.py and some of its own helper files.

I mispoke when i added a .py at the end of the include it does not have one.

So in my node class, all it does, is it prints hello -- i literally am just testing the ability to pull in different python modules into ros2.

My colcon build passes which is what i expect, but on the run of the node file, it fails on the line that is including the dstar_algorithm .

i import it like from dstar.dstar_algorithm import *

I have also tried: from dstar import dstar_algorithm from .dstar.dstar_algorithm import * import dstar_algorithm

and the list goes on. So i think i need to affect the setup.py or the package.xml not sure

Comment by veilkrand on 2021-03-04:
Have you been able to find a solution? Thanks

Answer 1

I think this question is also related so I recommend to check it out first: https://answers.ros.org/question/319867/ros2-colcon-build-not-getting-subfolders/

When ROS2 builds the src files with colcon, it puts the python files into workspace/install/lib/python3.8/site-packages/<package_name>. This folder is considered as the root of python packages in the given ROS 2 package.

If you can find the python files there, then you can use the following import statement: import .<file_name>. Note the . at the start of the import. You don't have to edit nothing else.

If you want to put the imported files into submodules, then you have to achieve that they show up in the workspace/install/lib/python3.8/site-packages/<package_name>/<submodule_name> folder. This can be done by replacing the packages argument in the setup.py file to the following: packages=["<package_name>", "<packages_name>/<submodule_name>"]. This way the import statement is import .<submodule_name>.<file_name>.

Example file structure to import with submodule:

workspace
|-> ...
|-> src
      |-> mypackage
             |-> mypackage
                    |-> submodules
                           |->__init__.py
                           |-> imported.py
                    |-> __init__.py
                    |-> main.py
             |-> resource
                    |-> ...
             |-> test
                    |-> ...
             |-> package.xml
             |-> setup.cfg
             |-> setup.py

setup.py:

from setuptools import setup

package_name = 'mypackage'
submodules = "mypackage/submodules"

setup(
    name=package_name,
    version='0.0.0',
    packages=[package_name, submodules],
    data_files=[
        ('share/ament_index/resource_index/packages',
            ['resource/' + package_name]),
        ('share/' + package_name, ['package.xml']),
    ],
    install_requires=['setuptools'],
    zip_safe=True,
    maintainer='TODO',
    maintainer_email='[email protected]',
    description='TODO: Package description',
    license='TODO: License declaration',
    tests_require=['pytest'],
    entry_points={
        'console_scripts': [
            'main = mypackage.main:main',
        ],
    },
)

main.py:

from .submodules.imported import function


def main():
    function()


if __name__ == '__main__':
    main()

imported.py:

def function():
    print("hello world")

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Originally posted by nagda9 with karma: 116 on 2021-05-29

This answer was ACCEPTED on the original site

Post score: 10

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Original comments

Comment by engro39 on 2021-09-15:
thank you so much. It works well.

Comment by xzkeee on 2022-04-04:
with dot. I can run as pkg, but cannot run directly from vscode, (without dot I can) is there a way to do both?

Comment by Bogdan on 2022-06-24:
I have the same problem, i can either run my nodes with a launch file, or directly from vscode but both of them are not working at the same time, i allways get an import related error e.g. module not found.

Comment by nagda9 on 2022-07-09:
Well, I think ROS 2 is still a bit quirky/clumsy in this regard. It was designed to be used in the "ROS" way: you have to build the workspace first, then you can run your program with the ros2 run command. As far as I know, it is not really compatible with "just running files". The build system is not allowing it to see the submodules in the usual way. I agree this is a disadvantage as it slows down development and makes things more complicated, but for now, this is the cost of the necessary build tool due to platform independentness. To correct this issue, either colcon should be changed, or ROS 2 should use a different build tool or build method.

Answer 2

Best start from the template ROS gives you:

ros2 pkg create --build-type ament_python --node-name dstar_node --library-name dstar_algorithm dstar

That gives you

dstar
├── dstar
│   ├── dstar_algorithm
│   │   └── __init__.py
│   ├── dstar_node.py
│   └── __init__.py
├── package.xml
├── resource
│   └── dstar
├── setup.cfg
├── setup.py
└── test
    ├── test_copyright.py
    ├── test_flake8.py
    └── test_pep257.py

You can then put a file, e.g. impl.py, with the implementation inside the dstar_algorithm directory and import it with from dstar.dstar_algorithm.impl import *.

Or you can replace the dstar_algorithm directory by a file dstar_algorithm.py that has the implementation and import with from dstar.dstar_algorithm import *.

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Originally posted by nnmm with karma: 351 on 2020-12-17

This answer was NOT ACCEPTED on the original site

Post score: 1

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Original comments

Comment by veilkrand on 2021-03-04:
--library-name parameter only creates and additional module folder. It doesn't fix any import/build issue

Comment by nagda9 on 2021-05-29:
This does not solve the problem.