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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 get_or_create_node(self, graph: BELGraph, node: BaseEntity) -> Optional[Node]: """Create an entry and object for given node if it does not exist."""

        sha512 = node.as_sha512()
        if sha512 in self.object_cache_node:
            return self.object_cache_node[sha512]

        node_model = self.get_node_by_hash(sha512)

        if node_model is not None:
            self.object_cache_node[sha512] = node_model
            return node_model

        node_model = Node._start_from_base_entity(node)

        namespace = node.get(NAMESPACE)
        if namespace is None:
            pass

        elif namespace in graph.namespace_url:
            url = graph.namespace_url[namespace]
            name = node[NAME]
            entry = self.get_namespace_entry(url, name)

            if entry is None:
                log.debug('skipping node with identifier %s: %s', url, name)
                return

            self.session.add(entry)
            node_model.namespace_entry = entry

        elif namespace in graph.namespace_pattern:
            name = node[NAME]
            pattern = graph.namespace_pattern[namespace]
            entry = self.get_or_create_regex_namespace_entry(namespace, pattern, name)

            self.session.add(entry)
            node_model.namespace_entry = entry

        else:
            log.warning("No reference in BELGraph for namespace: {}".format(node[NAMESPACE]))
            return

        if VARIANTS in node or FUSION in node:
            node_model.is_variant = True
            node_model.has_fusion = FUSION in node

            modifications = self.get_or_create_modification(graph, node)

            if modifications is None:
                log.warning('could not create %s because had an uncachable modification', node.as_bel())
                return

            node_model.modifications = modifications

        self.session.add(node_model)
        self.object_cache_node[sha512] = node_model
        return node_model