gh-156860: Fix exponential asyncio.print_call_graph() output

Lib/asyncio/graph.py

@@ -41,6 +41,7 @@ def _build_graph_for_future(
     future: futures.Future,
     *,
     limit: int | None = None,
+    seen: set[int] | None = None,
 ) -> FutureCallGraph:
     if not isinstance(future, futures.Future):
         raise TypeError(
@@ -68,9 +69,15 @@ def _build_graph_for_future(
         else:
             break
 
-    if future._asyncio_awaited_by:
+    if seen is None:
+        seen = set()
+
+    # gh-156860: "awaited by" is a DAG, not a tree. Expand each future once
+    if future._asyncio_awaited_by and id(future) not in seen:
+        seen.add(id(future))
         for parent in future._asyncio_awaited_by:
-            awaited_by.append(_build_graph_for_future(parent, limit=limit))
+            awaited_by.append(
+                _build_graph_for_future(parent, limit=limit, seen=seen))
 
     if limit is not None:
         if limit > 0:
@@ -170,8 +177,10 @@ def capture_call_graph(
 
     awaited_by = []
     if future._asyncio_awaited_by:
+        seen = {id(future)}
         for parent in future._asyncio_awaited_by:
-            awaited_by.append(_build_graph_for_future(parent, limit=limit))
+            awaited_by.append(
+                _build_graph_for_future(parent, limit=limit, seen=seen))
 
     if limit is not None:
         limit *= -1

Lib/test/test_asyncio/test_graph.py

@@ -434,6 +434,42 @@ async def main():
 
         self.assertTrue(stack_for_fut[1].startswith('* Future(id='))
 
+    async def test_build_graph_for_future_expands_dag_once(self):
+        # gh-156860: a future reachable by several paths is expanded once.
+        async def waits_for(*deps):
+            await asyncio.gather(*deps)
+
+        fut = asyncio.Future()
+        layer = [fut]
+        for _ in range(3):
+            layer = [asyncio.ensure_future(waits_for(*layer)) for _ in range(2)]
+        await asyncio.sleep(0)
+        captured = asyncio.format_call_graph(fut)
+
+        fut.set_result(None)
+        await asyncio.gather(*layer)
+
+        self.assertEqual(captured.count('* Task'), 10)
+
+    async def test_capture_call_graph_expands_dag_once(self):
+        # gh-156860
+        captured = None
+
+        async def waits_for(*deps):
+            await asyncio.gather(*deps)
+
+        async def root():
+            nonlocal captured
+            await asyncio.sleep(0)
+            captured = asyncio.format_call_graph()
+
+        layer = [asyncio.ensure_future(root())]
+        for _ in range(3):
+            layer = [asyncio.ensure_future(waits_for(*layer)) for _ in range(2)]
+        await asyncio.gather(*layer)
+
+        self.assertEqual(captured.count('* Task'), 13)
+
     async def test_capture_call_graph_positive_limit(self):
         captured = None
 

Misc/NEWS.d/next/Library/2026-09-03-01-32-46.gh-issue-156860.mPeXty.rst

@@ -0,0 +1,2 @@
+Fix exponential growth of :func:`asyncio.print_call_graph` output when
+several tasks await the same future.