class_docstr: A simple graph class
  
      
code: def add_node(self, n, attr=None):
      self.succ[n] = {}
      self.pred[n] = {}
      self.node[n] = attr
  
file_path: dipy/dipy/core/graph.py
label_desc: Use image node_id 2 for calling the Graph obj's underlying member method code with example usage: obj.add_node(n, attr) without return types