__init__.py

from types import ModuleType
import sys


class LazyModule(ModuleType):
    def __init__(self, name, doc, source):
        super().__init__(name, doc)
        self.__source = source
        self.__loaded = None

    def __initialize_load(self):
        if self.__loaded is None:
            module = ModuleType(self.__name__, self.__doc__)
            exec(self.__source, module.__dict__, module.__dict__)
            sys.modules[self.__name__] = module
            self.__loaded = module

    def __getattr__(self, name):
        if name.startswith('_LazyModule__'):
            return super().__getattr__(name)
        self.__initialize_load()
        return getattr(self.__loaded, name)

    def __setattr__(self, name, value):
        if name.startswith('_LazyModule__'):
            return super().__setattr__(name, value)
        self.__initialize_load()
        return setattr(self.__loaded, name, value)

    def __delattr__(self, name):
        if name.startswith('_LazyModule__'):
            return super().__delattr__(name)
        self.__initialize_load()
        return delattr(self.__loaded, name)

    def __dir__(self):
        self.__initialize_load()
        return self.__loaded.__dir__()


def submodule(fn):
    from inspect import getsource

    name = fn.__name__
    doc = fn.__doc__

    root = fn.__module__

    module_name = root + "." + name

    source = getsource(fn)

    lines = source.splitlines()

    for start, line in enumerate(lines):
        if line.lstrip().startswith('def '):
            break

    lines[start] = 'if True:'
    for i in range(start):
        lines[i] = ''

    source = '\n'.join(lines)

    module = LazyModule(module_name, doc, source)    

    sys.modules[module_name] = module

    return module


@submodule
def foo():
    hello = "WORLD"