Function wait_for_initialization 

pub(crate) fn wait_for_initialization()

If PyO3 is currently running Py_InitializeEx inside the Once guard, block until it completes. Needed because Py_InitializeEx sets the initialized flag in the interpreter to true before it finishes all its steps (in particular, before it imports site.py).

This must only be called after Py_IsInitialized() has returned true.

If the Once was never started (e.g. the interpreter was initialized externally, not through PyO3), call_once runs the empty closure and returns — this is fine because initialize() checks Py_IsInitialized() inside its closure and skips Py_InitializeEx if the interpreter is already running. If the Once is currently in progress (another thread is inside initialize()), call_once blocks until it completes.