Reject an empty custom timestep list with a clear ValueError

.ramp/empty-timesteps/architecture-history.json

@@ -0,0 +1,25 @@
+[
+  {
+    "decision": "REVISE",
+    "rationale": "Replacing an empty custom timestep list with [999] silently invents a one-step schedule the caller did not request. code_style.md forbids guessing user intent and silently correcting unsupported input; philosophy.md prefers a concise error over silent correction. 999 is also only meaningful when num_train_timesteps is 1000, so it is not a general default.",
+    "sources": [
+      ".ai/references/code_style.md",
+      "docs/source/en/conceptual/philosophy.md"
+    ],
+    "alternative": "Raise ValueError in DDPMScheduler.set_timesteps when timesteps is empty, after the existing mutually exclusive argument check and before timesteps[0] is read. Do not invent a default list.",
+    "task_sha256": "cf0af9c5d0cd003518d722c54d0951d63f15b68f68116e1696ad8aec2b6bd780",
+    "kind": "agent_assessment_not_human_approval"
+  },
+  {
+    "decision": "COMPATIBLE",
+    "rationale": "A local ValueError in DDPMScheduler.set_timesteps fits the existing custom-timestep checks in that method. Empty input is rejected after the mutual-exclusion error and before timesteps[0] is read, so valid lists, single-element lists, and num_inference_steps schedules stay unchanged. The parallel scheduler already copies this method; propagate the edit with the copy tool instead of adding a shared validation service.",
+    "sources": [
+      "src/diffusers/schedulers/scheduling_ddpm.py",
+      "src/diffusers/schedulers/scheduling_ddpm_parallel.py",
+      ".ai/references/code_style.md"
+    ],
+    "alternative": "",
+    "task_sha256": "cf0af9c5d0cd003518d722c54d0951d63f15b68f68116e1696ad8aec2b6bd780",
+    "kind": "agent_assessment_not_human_approval"
+  }
+]

.ramp/empty-timesteps/architecture.json

@@ -0,0 +1,12 @@
+{
+  "decision": "COMPATIBLE",
+  "rationale": "A local ValueError in DDPMScheduler.set_timesteps fits the existing custom-timestep checks in that method. Empty input is rejected after the mutual-exclusion error and before timesteps[0] is read, so valid lists, single-element lists, and num_inference_steps schedules stay unchanged. The parallel scheduler already copies this method; propagate the edit with the copy tool instead of adding a shared validation service.",
+  "sources": [
+    "src/diffusers/schedulers/scheduling_ddpm.py",
+    "src/diffusers/schedulers/scheduling_ddpm_parallel.py",
+    ".ai/references/code_style.md"
+  ],
+  "alternative": "",
+  "task_sha256": "cf0af9c5d0cd003518d722c54d0951d63f15b68f68116e1696ad8aec2b6bd780",
+  "kind": "agent_assessment_not_human_approval"
+}

.ramp/empty-timesteps/task.json

@@ -0,0 +1,51 @@
+{
+  "id": "empty-timesteps",
+  "title": "Reject an empty custom timestep list with a clear ValueError",
+  "request": "Make passing an empty custom timestep list produce a clear error while preserving valid inputs.",
+  "proposed_approach": "The engineer asked whether an empty list could be replaced with [999]. Treat that as a hypothesis and reject a silent default if library guidance prefers a clear error.",
+  "recipe": "scheduler-input-validation",
+  "editable_files": [
+    "src/diffusers/schedulers/scheduling_ddpm.py",
+    "src/diffusers/schedulers/scheduling_ddpm_parallel.py",
+    "tests/schedulers/test_scheduler_ddpm.py"
+  ],
+  "regression_test": "tests/schedulers/test_scheduler_ddpm.py::DDPMSchedulerTest::test_set_timesteps_empty_list_raises_value_error",
+  "baseline_error": "IndexError",
+  "criteria": [
+    {
+      "id": "C-EMPTY",
+      "text": "DDPMScheduler.set_timesteps and the copied DDPMParallelScheduler.set_timesteps raise ValueError when timesteps is an empty list; the existing both-argument conflict error still wins when both arguments are supplied.",
+      "tests": [
+        "tests/schedulers/test_scheduler_ddpm.py::DDPMSchedulerTest::test_set_timesteps_empty_list_raises_value_error"
+      ]
+    },
+    {
+      "id": "C-VALID",
+      "text": "A descending custom list and a single-element custom list keep their values on CPU when no device is requested.",
+      "tests": [
+        "tests/schedulers/test_scheduler_ddpm.py::DDPMSchedulerTest::test_set_timesteps_keeps_nonempty_custom_lists"
+      ]
+    },
+    {
+      "id": "C-MUTEX",
+      "text": "Passing num_inference_steps and timesteps together still raises the existing mutual-exclusion ValueError.",
+      "tests": [
+        "tests/schedulers/test_scheduler_ddpm.py::DDPMSchedulerTest::test_custom_timesteps_passing_both_num_inference_steps_and_timesteps"
+      ]
+    },
+    {
+      "id": "C-COUNT",
+      "text": "Calling set_timesteps with only num_inference_steps still builds a descending schedule of that length.",
+      "tests": [
+        "tests/schedulers/test_scheduler_ddpm.py::DDPMSchedulerTest::test_set_timesteps_num_inference_steps_still_works"
+      ]
+    }
+  ],
+  "exclusions": [
+    "Do not change scheduler numerical updates",
+    "Do not add a shared validation helper or new dependency",
+    "Do not silently replace an empty list with a default schedule",
+    "Do not publish or deploy"
+  ],
+  "base_sha": "0121a91f9d419ff7234c8a5923f82c244e6f1914"
+}

src/diffusers/schedulers/scheduling_ddpm.py

@@ -289,13 +289,16 @@ def set_timesteps(
             timesteps (`list[int]`, *optional*):
                 Custom timesteps used to support arbitrary spacing between timesteps. If `None`, then the default
                 timestep spacing strategy of equal spacing between timesteps is used. If `timesteps` is passed,
-                `num_inference_steps` must be `None`.
+                `num_inference_steps` must be `None`. The list must be non-empty.
 
         """
         if num_inference_steps is not None and timesteps is not None:
             raise ValueError("Can only pass one of `num_inference_steps` or `custom_timesteps`.")
 
         if timesteps is not None:
+            if len(timesteps) == 0:
+                raise ValueError("`timesteps` cannot be empty.")
+
             for i in range(1, len(timesteps)):
                 if timesteps[i] >= timesteps[i - 1]:
                     raise ValueError("`custom_timesteps` must be in descending order.")

src/diffusers/schedulers/scheduling_ddpm_parallel.py

@@ -304,13 +304,16 @@ def set_timesteps(
             timesteps (`list[int]`, *optional*):
                 Custom timesteps used to support arbitrary spacing between timesteps. If `None`, then the default
                 timestep spacing strategy of equal spacing between timesteps is used. If `timesteps` is passed,
-                `num_inference_steps` must be `None`.
+                `num_inference_steps` must be `None`. The list must be non-empty.
 
         """
         if num_inference_steps is not None and timesteps is not None:
             raise ValueError("Can only pass one of `num_inference_steps` or `custom_timesteps`.")
 
         if timesteps is not None:
+            if len(timesteps) == 0:
+                raise ValueError("`timesteps` cannot be empty.")
+
             for i in range(1, len(timesteps)):
                 if timesteps[i] >= timesteps[i - 1]:
                     raise ValueError("`custom_timesteps` must be in descending order.")

tests/schedulers/test_scheduler_ddpm.py

@@ -1,6 +1,6 @@
 import torch
 
-from diffusers import DDPMScheduler
+from diffusers import DDPMParallelScheduler, DDPMScheduler
 
 from .test_schedulers import SchedulerCommonTest
 
@@ -190,6 +190,37 @@ def test_custom_timesteps_too_large(self):
         ):
             scheduler.set_timesteps(timesteps=timesteps)
 
+    def test_set_timesteps_empty_list_raises_value_error(self):
+        for scheduler_class in (DDPMScheduler, DDPMParallelScheduler):
+            scheduler = scheduler_class(**self.get_scheduler_config())
+            with self.assertRaisesRegex(
+                ValueError, "Can only pass one of `num_inference_steps` or `custom_timesteps`."
+            ):
+                scheduler.set_timesteps(num_inference_steps=4, timesteps=[])
+            with self.assertRaisesRegex(ValueError, r"`timesteps` cannot be empty"):
+                scheduler.set_timesteps(timesteps=[])
+
+    def test_set_timesteps_keeps_nonempty_custom_lists(self):
+        scheduler_config = self.get_scheduler_config()
+        custom = [100, 87, 50, 1, 0]
+        scheduler = self.scheduler_classes[0](**scheduler_config)
+        scheduler.set_timesteps(timesteps=custom)
+        self.assertEqual(scheduler.timesteps.tolist(), custom)
+        self.assertEqual(scheduler.timesteps.device.type, "cpu")
+
+        scheduler = self.scheduler_classes[0](**scheduler_config)
+        scheduler.set_timesteps(timesteps=[0])
+        self.assertEqual(scheduler.timesteps.tolist(), [0])
+        self.assertEqual(scheduler.timesteps.device.type, "cpu")
+
+    def test_set_timesteps_num_inference_steps_still_works(self):
+        scheduler = self.scheduler_classes[0](**self.get_scheduler_config())
+        scheduler.set_timesteps(num_inference_steps=10)
+        self.assertEqual(len(scheduler.timesteps), 10)
+        timesteps = scheduler.timesteps.tolist()
+        self.assertEqual(timesteps, sorted(timesteps, reverse=True))
+        self.assertFalse(scheduler.custom_timesteps)
+
     def test_full_loop_with_noise(self):
         scheduler_class = self.scheduler_classes[0]
         scheduler_config = self.get_scheduler_config()