What is the output of clf.tree_.feature?

Question

I observed that scikit-learn clf.tree_.feature occasional return negative values. For example -2. As far as I understand clf.tree_.feature is supposed to return sequential order of the features. In case we have array of feature names ['feature_one', 'feature_two', 'feature_three'], then -2 would refer to feature_two. I am surprised with usage of negative index. In would make more sense to refer to feature_two by index 1. (-2 is reference convenient for human digestion, not for machine processing). Am I reading it correctly?

Update: Here is an example:

def leaf_ordering():
    X = np.genfromtxt('X.csv', delimiter=',')
    Y = np.genfromtxt('Y.csv',delimiter=',')
    dt = DecisionTreeClassifier(min_samples_leaf=10, random_state=99)
    dt.fit(X, Y)
    print(dt.tree_.feature)

Here are the files X and Y

Here is the output:

    [ 8  9 -2 -2  9  4 -2  9  8 -2 -2  0  0  9  9  8 -2 -2  9 -2 -2  6 -2 -2 -2
  2 -2  9  8  6  9 -2 -2 -2  8  9 -2  9  6 -2 -2 -2  6 -2 -2  9 -2  6 -2 -2
  2 -2 -2]

Answer 1

By reading the Cython source code for the tree generator we see that the -2's are just dummy values for the leaf nodes's feature split attribute.

Line 63

TREE_UNDEFINED = -2

Line 359

if is_leaf:
    # Node is not expandable; set node as leaf
    node.left_child = _TREE_LEAF
    node.right_child = _TREE_LEAF
    node.feature = _TREE_UNDEFINED
    node.threshold = _TREE_UNDEFINED