<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _InitializeGraph(self, os_name, artifact_list):
 """Creates the nodes and directed edges of the dependency graph. Args: os_name: String specifying the OS name. artifact_list: List of requested artifact names. """

    dependencies = artifact_registry.REGISTRY.SearchDependencies(
        os_name, artifact_list)
    artifact_names, attribute_names = dependencies

    self._AddAttributeNodes(attribute_names)
    self._AddArtifactNodesAndEdges(artifact_names)