Networkx MultiDiGraph inheritance: add_weighed_edges_from KeyError: None

Question

I tried to make PERT graph using MultiDiGraph. So, I tried to put a bool variable "_dirty" that checks whether the graph is deformed. However, the add_weighted_edges_from function does not work properly. Is the code wrong? Any help would be greatly appreciated.

class ClassName(nx.MultiDiGraph):
    # some code..

    def add_edge(self, *args, **kwargs):
        """ add one edge """
        self._dirty = True
        super().add_edge(*args, **kwargs)
        
    def add_edges_from(self, *args, **kwargs):
        """ add several edges """
        self._dirty = True
        super().add_edges_from(*args, **kwargs)  

    def add_weighted_edges_from(self, *args, **kwargs):
        """ add several weighted edges """
        self._dirty = True
        super().add_weighted_edges_from(*args, **kwargs)

g = ClassName()
g.add_weighted_edges_from([(1,2,5),(2,3,10)])

error i got:

KeyError                                  Traceback (most recent call last)
~\AppData\Local\Temp/ipykernel_15100/3885668308.py in <module>  
----> 1 g.add_weighted_edges_from([(1,2,5),(2,3,10)])    

~\AppData\Local\Temp/ipykernel_15100/2908507458.py in add_weighted_edges_from(self, *args, **kwargs)
     32         """  add several weighted edges """
     33         self._dirty = True
---> 34         super().add_weighted_edges_from(*args, **kwargs)
     35 
     36     def remove_node(self, *args, **kwargs):    

~\anaconda3\envs\pytorch_env\lib\site-packages\networkx\classes\graph.py in add_weighted_edges_from(self, ebunch_to_add, weight, **attr)  
    986         >>> G.add_weighted_edges_from([(0, 1, 3.0), (1, 2, 7.5)])
    987           
--> 988         self.add_edges_from(((u, v, {weight: d}) for u, v, d in ebunch_to_add), **attr)  
    989   
    990     def remove_edge(self, u, v):    

~\AppData\Local\Temp/ipykernel_15100/2908507458.py in add_edges_from(self, *args, **kwargs)  
     27         """ add several edges """  
     28         self._dirty = True  
---> 29         super().add_edges_from(*args, **kwargs)  
     30  
     31     def add_weighted_edges_from(self, *args, **kwargs):    

~\anaconda3\envs\pytorch_env\lib\site-packages\networkx\classes\multigraph.py in add_edges_from(self, ebunch_to_add, **attr)  
    571                 key = dd  # ne == 3 with 3rd value not dict, must be a key  
    572             key = self.add_edge(u, v, key)  
--> 573             self[u][v][key].update(ddd)  
    574             keylist.append(key)  
    575         return keylist    

~\anaconda3\envs\pytorch_env\lib\site-packages\networkx\classes\coreviews.py in __getitem__(self, key)  
     52  
     53     def __getitem__(self, key):  
---> 54         return self._atlas[key]  
     55  
     56     def copy(self):    

KeyError: None

Answer 1

You missed that add_edge returns the key in a Multi(Di)Graph. Adding the return resolves your issue:

class RailwayGraph(nx.MultiDiGraph):
    # some code..

    def add_edge(self, *args, **kwargs):
        """ add one edge """
        self._dirty = True
        return super().add_edge(*args, **kwargs)

    def add_edges_from(self, *args, **kwargs):
        """ add several edges """
        self._dirty = True
        super().add_edges_from(*args, **kwargs)

    def add_weighted_edges_from(self, *args, **kwargs):
        """ add several weighted edges """
        self._dirty = True
        super().add_weighted_edges_from(*args, **kwargs)


rg = RailwayGraph()
rg.add_weighted_edges_from([(1, 2, 5), (2, 3, 10)])