Not Fitted Error when using Sklearn's graphviz

(Only going by the docs; no personal experience)

You are trying to plot some DecisionTree, using a function which signature reads:

sklearn.tree.export_graphviz(decision_tree, ...)

but you are passing a RandomForest, which is an ensemble of trees.

That's not going to work!

Going deeper, the code internally for this is here:

check_is_fitted(decision_tree, 'tree_')

So this is asking for the attribute tree_ of your DecisionTree, which exists for a DecisionTreeClassifier.

This attribute does not exist for a RandomForestClassifier! Therefore the error.

The only thing you can do: print every DecisionTree within your RandomForest ensemble. For this, you need to traverse random_forest.estimators_ to get the underlying decision-trees!

Like the other answer said, you cannot do this for a forest, only a single tree. You can, however, graph a single tree from that forest. Here's how to do that:

forest_clf = RandomForestClassifier()
forest_clf.fit(X_train, y_train)
tree.export_graphviz(forest_clf.estimators_[0], out_file='tree_from_forest.dot')
(graph,) = pydot.graph_from_dot_file('tree_from_forest.dot')
graph.write_png('tree_from_forest.png')

Unfortunately, there's no easy way to graph the "best" tree or an overall ensemble tree from your forest, just a random example tree.