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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 init_graph(self):
 """ Initialize graph Load all nodes and set dependencies. To avoid errors about missing nodes all nodes get loaded first before setting the dependencies. """

        self._graph = Graph()

        # First add all nodes
        for key in self.loader.disk_fixtures.keys():
            self.graph.add_node(key)

        # Then set dependencies
        for key, fixture in self.loader.disk_fixtures.items():
            for dependency in fixture.dependencies:
                self.graph.add_dependency(key, dependency)