Question

I have a networkx graph.
I am adding nodes by adding edges

G.add_edge(route[i-1],route[i]);

Now once the node is created by directly adding edges,
I add a list named

G.node[route[i]]['position'] = list()

and I append positions to it when I get same nodes again and again

G.node[route[i]]['position'].append( i - 3 )

Now when I want to append how do I check whether the list exist?
does doing

G.node[route[i]]['position'] = list()

clear the list of already existing elements?

edit—– my earlier question was confusing
I want to keep adding to the list
but I cant append unless a list exists, right?
So I have to do do
G.node[route[i]]['position'] = list() in my loop
So next time when I want to add to the same list in another loop instance how do I know that a list exists for G.node[route[i]]['position'] and I dont have to create it again.

edit—–
I think my list itself is a key here
so I did

                    if not 'position' in G.node[route[i]]:

and it works

Answer 1

G.node[route[i]]['position'] = list() will leave the slot G.node[route[i]]['position'] holding an empty list, but it will not affect the list that it previously held, which other objects may have a reference to.

Instead, use: del l[:] to empty the list.

If you want to have a list automatically created, use collections.defaultdict to have newly created entries default to a list.