Fixed #22531 and added tests for tree.Node

While Node class has a useful `__str__`, its `__repr__` is not that
useful. Added a `__repr__` that makes use of the current `__str__`.
This is especially useful since the more popular `Q` class inherits
`tree.Node`. Also created new tests that cover most of `Node` class
functionality.

django/utils/tree.py

@@ -48,6 +48,9 @@ def __str__(self):
         return '(%s: %s)' % (self.connector, ', '.join([str(c) for c in
                 self.children]))
 
+    def __repr__(self):
+        return "<%s: %s>" % (self.__class__.__name__, self)
+
     def __deepcopy__(self, memodict):
         """
         Utility method used by copy.deepcopy().

tests/utils_tests/test_tree.py

@@ -0,0 +1,56 @@
+import unittest
+
+import copy
+from django.utils.tree import Node
+
+class NodeTests(unittest.TestCase):
+    def setUp(self):
+        self.node1_children = [('a', 1), ('b', 2)]
+        self.node1 = Node(self.node1_children)
+        self.node2 = Node()
+
+    def test_str(self):
+        self.assertEqual(str(self.node1), "(DEFAULT: ('a', 1), ('b', 2))")
+        self.assertEqual(str(self.node2), "(DEFAULT: )")
+
+    def test_repr(self):
+        self.assertEqual(repr(self.node1),
+                         "<Node: (DEFAULT: ('a', 1), ('b', 2))>")
+        self.assertEqual(repr(self.node2), "<Node: (DEFAULT: )>")
+
+    def test_len(self):
+        self.assertEqual(len(self.node1), 2)
+        self.assertEqual(len(self.node2), 0)
+
+    def test_bool(self):
+        self.assertTrue(self.node1)
+        self.assertFalse(self.node2)
+
+    def test_contains(self):
+        self.assertIn(('a', 1), self.node1)
+        self.assertNotIn(('a', 1), self.node2)
+
+    def test_add(self):
+        # start with the same children of node1 then add an item
+        node3 = Node(self.node1_children)
+        node3_added_child = ('c', 3)
+        # add() returns the added data
+        self.assertEqual(node3.add(node3_added_child, Node.default),
+                         node3_added_child)
+        # we added exactly one item, len() should reflect that
+        self.assertEqual(len(self.node1)+1, len(node3))
+        self.assertEqual(str(node3), "(DEFAULT: ('a', 1), ('b', 2), ('c', 3))")
+
+    def test_negate(self):
+        # negated is False by default
+        self.assertFalse(self.node1.negated)
+        self.node1.negate()
+        self.assertTrue(self.node1.negated)
+        self.node1.negate()
+        self.assertFalse(self.node1.negated)
+
+    def test_deepcopy(self):
+        node4 = copy.copy(self.node1)
+        node5 = copy.deepcopy(self.node1)
+        self.assertIs(self.node1.children, node4.children)
+        self.assertIsNot(self.node1.children, node5.children)