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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 topological_sort(self):
 """ Returns a topological ordering of the DAG. Returns: list: A list of topologically sorted nodes in the graph. Raises: ValueError: Raised if the graph is not acyclic. """

        graph = self.graph

        in_degree = {}
        for u in graph:
            in_degree[u] = 0

        for u in graph:
            for v in graph[u]:
                in_degree[v] += 1

        queue = deque()
        for u in in_degree:
            if in_degree[u] == 0:
                queue.appendleft(u)

        sorted_graph = []
        while queue:
            u = queue.pop()
            sorted_graph.append(u)
            for v in sorted(graph[u]):
                in_degree[v] -= 1
                if in_degree[v] == 0:
                    queue.appendleft(v)

        if len(sorted_graph) == len(graph):
            return sorted_graph
        else:
            raise ValueError('graph is not acyclic')