# Rules for programming with Python in ROS2
## File Structure
```
colcon_workspace  
      |  
      |- build     (holds build files)
      |- install   (holds build elements)
      |- src       (contains your source code - this is where you work)
```

## Colcon Build
Performing a `colcon build` is neccessary after every change to the `src` directory. This must be performed in the `root` of the _colcon_ workspace (eg. `cd ~/colcon_ws`).

In order to avoid this procedure after every change made to a Python script you must run `colcon build --symlink-install` once. This links the files created in `/build` and `/install` to your source files in `/src`. However, after creating a new Python script this procedure must be repeated in order to also link this new script.

## Source Directory
The source directory holds your ROS2 packages as further directories. In this example the `src` directory contains the packages `pubsub` and `pubsub_msg`:
```
colcon_workspace/
      |
      |- src/
          |
          |- pubsub/
          |- pubsub_msg/
```
The `src` directory is also the place where you create your new packages using `ros2 pkg create ...` and not in the root of the workspace.  

The next chapters will describe the content of the files inside a package directory. However, the occuring files depend on the type of package (`ament_cmake` for C++ scripts or `ament_python` for Python scripts). This is described in detail [here](https://docs.ros.org/en/foxy/Tutorials/Creating-Your-First-ROS2-Package.html). The following chapters only describe Python packages.


## setup.py
The file setup.py is used to create entry points, meaning you link your python scripts to names which you can be called through `ros2 run ...`. In order for this to work your python scripts must be written using the following mechanism
```python
def main():
  # main code

if __name__ == "__main__":
  main()
```
in which you put all of your code inside `main()`. This prevents any code from being run, if this script is not run as main.  

Linking your scripts to names is done inside the file _setup.py_ by defining entry points:
```python
entry_points={
   'console_scripts': [
       'talker = pubsub.talker:main',
       'listener = pubsub.listener:main'
   ],
},
```
The _setup.py_-excerp above links the the function `main()` from the scripts _talker.py_ and _listener.py_ from inside `/pubsub` to the names _talker_ and _listener_. This way they can be called using `ros2 run pubsub talker` or `ros2 run pubsub listener`.  
  
From this you can see that you need to follow the Python programming style of creating a _main()_ procedure, in order for the entry points to work.