dev(hansbug): test delayed flatten

test/tree/tree/test_flatten.py

 import pytest
 
-from treevalue.tree import TreeValue, raw, flatten, unflatten, flatten_values, flatten_keys
+from treevalue.tree import TreeValue, raw, flatten, unflatten, flatten_values, flatten_keys, delayed
 
 
 class MyTreeValue(TreeValue):
@@ -22,15 +22,37 @@ class TestTreeTreeFlatten:
             (('d', 'y'), 4)
         ]
 
+        t1 = TreeValue({
+            'a': delayed(lambda: t.a),
+            'b': delayed(lambda: t.b),
+            'c': delayed(lambda: t.c),
+            'd': delayed(lambda: t.d),
+        })
+        flatted = sorted(flatten(t1))
+        assert flatted == [
+            (('a',), 1),
+            (('b',), 2),
+            (('c',), {'x': 3, 'y': 4}),
+            (('d', 'x'), 3),
+            (('d', 'y'), 4)
+        ]
+
     def test_flatten_values(self):
         t = TreeValue({'a': 1, 'b': 5, 'c': {'x': 3, 'y': 4}, 'd': {'x': 3, 'y': 4}})
-
         flatted_values = sorted(flatten_values(t))
         assert flatted_values == [1, 3, 3, 4, 4, 5]
 
+        t1 = TreeValue({
+            'a': delayed(lambda: t.a),
+            'b': delayed(lambda: t.b),
+            'c': delayed(lambda: t.c),
+            'd': delayed(lambda: t.d),
+        })
+        flatted_values = sorted(flatten_values(t1))
+        assert flatted_values == [1, 3, 3, 4, 4, 5]
+
     def test_flatten_keys(self):
         t = TreeValue({'a': 1, 'd': {'x': 3, 'y': 4}, 'e': raw({'x': 3, 'y': 4}), 'b': 5, 'c': {'x': 3, 'y': 4}})
-
         flatted_keys = sorted(flatten_keys(t))
         assert flatted_keys == [
             ('a',),
@@ -42,6 +64,24 @@ class TestTreeTreeFlatten:
             ('e',),
         ]
 
+        t1 = TreeValue({
+            'a': delayed(lambda: t.a),
+            'b': delayed(lambda: t.b),
+            'c': delayed(lambda: t.c),
+            'd': delayed(lambda: t.d),
+            'e': delayed(lambda: t.e),
+        })
+        flatted_keys = sorted(flatten_keys(t1))
+        assert flatted_keys == [
+            ('a',),
+            ('b',),
+            ('c', 'x',),
+            ('c', 'y',),
+            ('d', 'x',),
+            ('d', 'y',),
+            ('e',),
+        ]
+
     def test_unflatten(self):
         flatted = [
             (('a',), 1),