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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _lookup(cls: str) -> LdapObjectClass: """ Lookup module.class. """

    if isinstance(cls, str):
        module_name, _, name = cls.rpartition(".")
        module = importlib.import_module(module_name)
        try:
            cls = getattr(module, name)
        except AttributeError:
            raise AttributeError("%s reference cannot be found" % cls)
    return cls