Add example

synapse/util/caches/treecache.py

@@ -145,6 +145,20 @@ def iterate_tree_cache_items(key, value):
     """Helper function to iterate over the leaves of a tree, i.e. a dict of that
     can contain dicts.
 
+    The provided key is a tuple that will get prepended to the returned keys.
+
+    Example:
+
+        cache = TreeCache()
+        cache[(1, 1)] = "a"
+        cache[(1, 2)] = "b"
+        cache[(2, 1)] = "c"
+
+        tree_node = cache.get((1,))
+
+        items = iterate_tree_cache_items((1,), tree_node)
+        assert list(items) == [((1, 1), "a"), ((1, 2), "b")]
+
     Returns:
         A generator yielding key/value pairs.
     """