py_import_example - ROS2 Humble example package on how to import custom python modules

This repo was made to solve an error ModuleNotFoundError: No module named 'foo' by better understanding how installation process and import works

py_import_example - ROS2 Humble example package on how to import custom python modules

Extra info

https://stackoverflow.com/questions/57426715/import-modules-in-package-in-ros2

Main notes

To test this code git clone as py_import_example package or create package from scratch How to create such package from scratch

Package tree structure

src/py_import_example
├── LICENSE
├── package.xml
├── py_import_example
│   ├── file1.py  <--------- import module
│   ├── __init__.py
│   ├── my_node.py  <------- main node
│   └── subfolder
│       ├── file2.py  <----- import module in a subfolder
├── README
├── resource
│   └── py_import_example
├── setup.cfg
└── setup.py  <------------- setup.py

How to import

In my_node.py there are multiple methods to import python modules

# To import module function
from .file1 import fun1  # Notice dot before module name ".file1"
from .subfolder.file2 import fun2

# To import whole module
from py_import_example import file1  # Notice "from <package_name>"
from py_import_example.subfolder import file2

# Same as previous
import py_import_example.file1 as file1
import py_import_example.subfolder.file2 as file2

Note the dot in from .file1 import fun1 it is relative import it tells Python to look for the module in the same directory as the module that contains this import statement
Meanwhile from file1 import fun1 is absolute import which searches for python modules in Python path.

Note that such imports won't work (at least in this package)

# import .file1  # NOT VALID
# import .py_import_example.file1  # NOT VALID

setup.py should be modified just for my_node.py. No modifications needed for file1.py and file2.py as those modules are not nodes and will only be imported.

entry_points={
    'console_scripts': [
        'my_node = py_import_example.my_node:main',

Where are python files installed

Python nodes installed in install/ directory

~/ros2_ws/install/py_import_example/lib/py_import_example/my_node

Python modules seems to be install in build/ directory which is referenced in install/ .egg-link file

cat ~/ros2_ws/install/py_import_example/lib/python3.10/site-packages/py-import-example.egg-link
# Out: ~/ros2_ws/build/py_import_example
~/ros2_ws/build/py_import_example/py_import_example/file1.py

How to create such package from scratch

Create ROS2 workspace or cd to src of existing workspace

# cd to ROS2 workspace

Create ament_python package

ros2 pkg create py_import_example --license Apache-2.0 --build-type ament_python --dependencies rclpy

Create python modules/files

cd py_import_example/py_import_example
touch my_node.py file1.py
mkdir subfolder
cd subfolder
touch file2.py
# Write source code to .py files

my_node.py

def main():
    print("Import method 1")
    from .file1 import fun1
    from .subfolder.file2 import fun2
    fun1()
    fun2()
    
    print("Import method 2")
    from py_import_example import file1
    from py_import_example.subfolder import file2
    file1.fun1()
    file2.fun2()

if __name__ == '__main__':
    main()

file1.py (node in same dir as my_node.py)

def fun1():
    print("file1.py")

file2.py (node in sufolder)

def fun2():
    print("file2.py")

In setup.py add 'my_node = py_import_example.my_node:main', for ROS2 node. No modifications needed for file1.py and file2.py as those modules are not nodes and will only be imported.

entry_points={
    'console_scripts': [
        'my_node = py_import_example.my_node:main',
    ],
},

Build

cd ..  # cd to src directory
colcon build --symlink-install

Run

ros2 run py_import_example my_node

#Expected output

Import method 1
file1.py
file2.py
Import method 2
file1.py
file2.py

Combinacijus/py_import_example