While loop DFS implementation

hamilton/graph.py

@@ -1090,13 +1090,19 @@ def directional_dfs_traverse(
         nodes = set()
         user_nodes = set()
 
-        def dfs_traverse(node: node.Node):
-            nodes.add(node)
-            for n in next_nodes_fn(node):
-                if n not in nodes:
-                    dfs_traverse(n)
-            if node.user_defined:
-                user_nodes.add(node)
+        def dfs_traverse_iterative(start_node: node.Node):
+            """Iterative DFS to avoid recursion depth limits with large DAGs."""
+            stack = [start_node]
+            while stack:
+                n = stack.pop()
+                if n in nodes:
+                    continue
+                nodes.add(n)
+                if n.user_defined:
+                    user_nodes.add(n)
+                for next_n in next_nodes_fn(n):
+                    if next_n not in nodes:
+                        stack.append(next_n)
 
         missing_vars = []
         for var in starting_nodes:
@@ -1107,7 +1113,7 @@ def dfs_traverse(node: node.Node):
                     # if it's not in the runtime inputs, it's a properly missing variable
                     missing_vars.append(var)
                 continue  # collect all missing final variables
-            dfs_traverse(self.nodes[var])
+            dfs_traverse_iterative(self.nodes[var])
         if missing_vars:
             missing_vars_str = ",\n".join(missing_vars)
             raise ValueError(f"Unknown nodes [{missing_vars_str}] requested. Check for typos?")

tests/test_graph.py

@@ -17,6 +17,7 @@
 
 import inspect
 import pathlib
+import sys
 import uuid
 from itertools import permutations
 from typing import List
@@ -1479,3 +1480,41 @@ def test_display_name_list_value_uses_first_element():
     assert "First Name" in dot_string
     # The function name should NOT appear since display_name is set
     assert "<b>node_with_list_display_name</b>" not in dot_string
+
+
+def test_get_upstream_nodes_large_chain_no_recursion_error():
+    """Regression test: get_upstream_nodes with only final_node on a large chain DAG.
+
+    A recursive DFS would exceed Python's recursion limit (~1000) when traversing
+    a long dependency chain from a single final node. This test verifies that
+    the iterative DFS in directional_dfs_traverse handles large DAGs correctly.
+
+    Chain size is chosen to exceed recursion limit: 1200 nodes > 1000.
+    """
+    from hamilton import ad_hoc_utils
+    from hamilton import function_modifiers as fm
+
+    def step(prev: float) -> float:
+        """Single step in a linear chain."""
+        return prev + 1.0
+
+    # Build a linear chain: node_0 -> node_1 -> ... -> node_N
+    chain_size = sys.getrecursionlimit() + 200  # Exceeds recursion limit
+    config = {}
+    for i in range(chain_size):
+        prev = f"node_{i - 1}" if i > 0 else 0.0
+        config[f"node_{i}"] = {
+            "prev": fm.source(prev) if i > 0 else fm.value(0.0),
+        }
+    decorated = fm.parameterize(**config)(step)
+    module = ad_hoc_utils.create_temporary_module(decorated, module_name="large_chain")
+
+    fg = graph.FunctionGraph.from_modules(module, config={})
+    final_node = f"node_{chain_size - 1}"
+
+    # This would raise RecursionError with recursive DFS
+    nodes, user_nodes = fg.get_upstream_nodes([final_node])
+
+    assert len(nodes) == chain_size
+    assert len(user_nodes) == 0
+    assert all(fg.nodes[f"node_{i}"] in nodes for i in range(chain_size))