[Scons-users] Incorrect diagnostics output when calling a Python function as custom action

[Andreas Heyer]

Hello devs and users of SCons,

my use of SCons is only through PlatformIO inside of VS Code. I don't have the time to discover how to make a suitable standalone example to show the small error. So please don't be rude because I only copied my bug report for PlatformIO (https://github.com/platformio/platformio-core/issues/4573) over to this project.


Configuration:

Operating system: Windows 10 x64
PlatformIO Version (platformio --version): 3.1.1 (VS Code extension)


Description of problem:

There is incorrect diagnostic output during the build process if a custom action is invoked via a custom Python script. This output can lead to false assumptions of the creator of the custom script because claimed action and in reality performed action do not match. In my case I copied the wrong file in my custom action because the diagnostic output showed the wrong order of parameters (source and target interchanged) albeit strictly sticking to the docs I could succeeded from the beginning.


Steps to Reproduce:

1. Add a custom Python script to the project's platformio.ini with "extra_scripts = post:myscript.py" under an environment section
2. Create an action function "action_func" with signature "action_func(source, target, env)" in the Python file myscript.py
3. Register this function as action for a target "generated_file_to_watch", e.g. "env.AddPostAction("generated_file_to_watch", action_func)"
4. Build the project
5. Watch for an output line saying "action_func(["generated_file_to_watch"], ["source_file"])".
    The list of source files (parameter source) depends on your choice of the generated file to watch (parameter target).


Actual Results:

Output line of
	action_func(["generated_file_to_watch"], ["source_file"])


Expected Results:

Output line of
	action_func(["source_file"], ["generated_file_to_watch"])

Because this is the order the signature of action_func demands according to PlatformIO's advanced scripting documentation. And in fact it's the order the function is called.