It looks simple and easy to understand.

To achieve lazy import without breaking backward compatibility,
I want to add one more rule:  When package defines both of __getattr__ and __all__, automatic import of submodules are disabled (sorry, I don't have pointer to specification about this behavior).

For example, some modules depends on email.parser or email.feedparser.
But since email/ uses __all__, all submodules
are imported eagerly.


Changing __all__ will break backward compatibility.
With __getattr__, this can be lazy import:

import importlib

def __getattr__(name):
    if name in __all__:
        return importlib.import_module("." + name, __name__)
    raise AttributeError(f"module {__name__!r} has no attribute {name!r}")