Update 'non-ros-files/Programming_Rules.md'

non-ros-files/Programming_Rules.md

@@ -29,10 +29,17 @@ The next chapters will describe the content of the files inside a package direct
 
 
 ## setup.py
-The file `<colcon_ws>/<package_dir>/setup.py` defines how you can call your scripts using a self defined in the command line. This results in commands such as  
+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
-`ros2 run pubsub talker`  
+```python
-`ros2 run pubsub listener`.  
+def main():
-This is done by linking the correct Python scripts to the names _talker_ and _listener_ using the file _setup.py_, eg.  
+  # 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': [
@@ -40,5 +47,7 @@ entry_points={
        'listener = pubsub.listener:main'
    ],
 },
-```  
+```
-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.
+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.