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/__init__.py uses __all__, all submodules

are imported eagerly.

See https://github.com/python/cpython/blob/master/Lib/email/__init__.py#L7-L25

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}")

Regards,