Add pythonization for freezing class attribute and preventing creating new attrubes by mistake

[m-fila]

BEGINRELEASENOTES

• Added pythonization for "freezing" class disallowing setting non-existent attributes

ENDRELEASENOTES

Pythonization proposed in key4hep/EDM4hep#357. Adding new attributes to existing objects is a feature in python. Unfortunately in the context of podio this is a very error prone behaviour that already lead to some bugs in the past.

For instance a typo in attribute name will silently create new attribute. With this pythonization an exception will be thrown
The error message is consistent with __slots__ which is conventionally used in standard python to construct classes with frozen attributes

[hegner]

@m-fila - always on the wish list and you did it - great! :-)

[tmadlener]

Just for my understanding. This is still opt-in for generated EDMs, right? And I only don't see the "opting in" in the diff of this PR, because for podio tests we already have opted in with the other pythonizations?

Is it possible to selectively opt in to the pythonizations?

[m-fila]

Yes, it's opt-in. That test opted for all the pythonizations in podio for namespace/module nsp

podio/python/podio/test_CodeGen.py

Lines 8 to 12 in 27e92d5

	from pythonizations import load_pythonizations # pylint: disable=import-error
	# load all available pythonizations to the classes in a namespace
	# loading pythonizations changes the state of cppyy backend shared by all the tests in a process
	load_pythonizations("nsp")

Loading specific pythonization is also supported

from  podio.pythonizations import freeze_class

freeze_class.FreezeClassPythonizer.register("edm4hep")

The docs are already there https://key4hep.web.cern.ch/podio/python.html#pythonizations 😉

[tmadlener]

The docs are already there https://key4hep.web.cern.ch/podio/python.html#pythonizations 😉

😂😂 you would think people look there first before asking stupid questions ;)

Thanks for the clarifications.