error is raised when I try to remove elements (pydot objects) from a python list

I am writing an algorithm to represent regression trees, using pydot module (an interface to Graphviz's Dot language). In the algorithm, lists of Edges and Nodes are made, and then they are represented - that is working fine.

But in some specific situations, I need to remove some of the Edges and Nodes, and that's where I am getting stuck. Here is part of the code:

import pydot
graph = pydot.Dot(graph_type='graph')

link4 = pydot.Edge(node10, node21, label=etiquetas[3])
link5 = pydot.Edge(node11, node22, label=etiquetas[4])
lista_links = [link4, link5]

# if some conditions are verified, then:
lista_links.remove(link5)

for link in lista_links:
graph.add_edge(link)
graph.write_png('teste.png')

I was expecting this code to work without any problem, but I get an error, saying:

Att开发者_如何转开发ributeError: 'NoneType' object has no attribute 'get_top_graph_type'

My only idea is, instead of removing the Nodes and Edges in some specific situations, to change the code and add only the Nodes and Edges after I define all the specific situations. But that would be a lot more work... (The code is much bigger than what I've shown you, and I have several specific situations that need to be considered).

I am curious why python behaves like this... Can somebody explain that to me, or give me any idea on how to change this behavior?

Thanks in advance, Carla

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At the surface, it seems that the problem is in Edges or Nodes without parent graph. So, the overall solution would be: do not allow nodes and edges hang around, always attach them to graph,and then remove from graph as needed.