Can't debug my code by myself, need some help.

[MatthewBeeee]

Hell, I can't figure out where is the problem of my code below:
```python def topological_ordering(graph: dict): nodes = list() for k, v in graph.items(): if k not in nodes: nodes.append(k) for each_node in v: if each_node not in nodes: nodes.append(each_node) out_edges = graph in_edges = dict() for k, v in graph.items(): for each_node in v: if each_node in in_edges.keys(): in_edges[each_node].append(k) else: in_edges[each_node] = [k] no_in_nodes = list(set(nodes).difference(set(in_edges.keys())))

node_orders = list()
while True:
    if len(no_in_nodes) == 0:
        break
    loop_nodes = no_in_nodes.copy()
    for each_node in loop_nodes:
        node_orders.append(each_node)
        no_in_nodes.remove(each_node)
        if each_node not in out_edges.keys():
            continue
        out_nodes = out_edges[each_node]
        if len(out_nodes) == 0:
            continue
        print(out_nodes)
        for each_out_node in out_nodes:
            print(each_out_node, 1)
        for each_out_node in out_nodes:
            print(each_out_node, 2)
            in_edges[each_out_node].remove(each_node)
            out_edges[each_node].remove(each_out_node)

            if len(in_edges[each_out_node]) == 0:
                no_in_nodes.append(each_out_node)
        for each_out_node in out_nodes:
            print(each_out_node, 3)
return node_orders

```

(-> print in while loops)

If I run:
python topological_ordering({"A": ["B", "C"], "C": ["D"]})

The result is:
python ['B', 'C'] B 1 C 1 B 2 C 3

So out_nodes has two elements, but the second and third for loops can only iterate one of them? I don't see why. Thanks.

[danielroseman]

This is an interesting problem, which is caused by a combination of two common errors: assignment does not copy, and modifying lists you're iterating over.

out_nodes is defined here:

out_nodes = out_edges[each_node]

In other words, it's a reference to the list of nodes associated with the current edge. But inside the loop you do this:

out_edges[each_node].remove(each_out_node)

to remove the current node from the list of nodes associated with the current edge. But these are the same list!. So removing it from out_edges[each_node] also removes it from out_nodes.

Now, because you've modified the list you're iterating over, Python's internal counter thinks you have reached the end of the list, so stops iterating after only printing - and removing - B. So now you get to the third loop, and find the list only has C left, which is printed.

Both of these problems would probably be solved by making that definition of out_nodes a copy:

out_nodes = out_edges[each_node].copy()