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While seing if it was worth making isinstance work with super(), I came up with this at Antoine's suggestion of using a proxy instead of a mixin:
class LazyProxy(importlib.abc.Loader):
 def __init__(self, loader):
 self.loader = loader
 def __call__(self, *args, **kwargs):
 self.args = args
 self.kwargs = kwargs
 return self
 def load_module(self, fullname):
 # XXX ignoring sys.modules details, e.g. if module already existed.
 lazy_module = LazyModule(fullname, proxy=self, name=fullname)
 sys.modules[fullname] = lazy_module
 return lazy_module
class LazyModule(types.ModuleType):
 def __init__(*args, proxy, name, **kwargs):
 self.__proxy = proxy
 self.__name = name
 super().__init__(*args, **kwargs)
 def __getattribute__(self, attr):
 self.__class__ = Module
 state = self.__dict__.copy()
 loader = self.__proxy.loader(*self.proxy.args, **self.proxy.kwargs)
 # XXX ignoring sys.modules details, e.g. removing module on load
failure.
 loader.load_module(self.__name)
 self.__dict__.update(state)
 return getattr(module, attr)