edtlib: add Node.child_index()

It can be useful to know what the index of a particular child is in
the list of nodes. Add a a helper for computing that and some test
cases.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>

scripts/dts/python-devicetree/src/devicetree/edtlib.py

@@ -724,6 +724,21 @@ class Node:
         return OrderedDict((name, self.edt._node2enode[node])
                            for name, node in self._node.nodes.items())
 
+    def child_index(self, node):
+        """Get the index of *node* in self.children.
+        Raises KeyError if the argument is not a child of this node.
+        """
+        if not hasattr(self, '_child2index'):
+            # Defer initialization of this lookup table until this
+            # method is callable to handle parents needing to be
+            # initialized before their chidlren. By the time we
+            # return from __init__, 'self.children' is callable.
+            self._child2index = OrderedDict()
+            for index, child in enumerate(self.children.values()):
+                self._child2index[child] = index
+
+        return self._child2index[node]
+
     @property
     def required_by(self):
         "See the class docstring"

scripts/dts/python-devicetree/tests/test.dts

@@ -310,7 +310,7 @@
 	};
 
 	//
-	// For testing Node.parent and Node.children
+	// For testing hierarchy.
 	//
 
 	parent {

scripts/dts/python-devicetree/tests/test_edtlib.py

@@ -158,6 +158,20 @@ def test_hierarchy():
 
     assert edt.get_node("/parent/child-1").children == {}
 
+def test_child_index():
+    '''Test Node.child_index.'''
+    with from_here():
+        edt = edtlib.EDT("test.dts", ["test-bindings"])
+
+    parent, child_1, child_2 = [edt.get_node(path) for path in
+                                ("/parent",
+                                 "/parent/child-1",
+                                 "/parent/child-2")]
+    assert parent.child_index(child_1) == 0
+    assert parent.child_index(child_2) == 1
+    with pytest.raises(KeyError):
+        parent.child_index(parent)
+
 def test_include():
     '''Test 'include:' and the legacy 'inherits: !include ...' in bindings'''
     with from_here():