fix(trainer): handle missing Podman container filters

kubeflow/trainer/backends/container/adapters/podman.py

@@ -209,6 +209,7 @@ def get_container_ip(self, container_id: str, network_id: str) -> str | None:
 
     def list_containers(self, filters: dict[str, list[str]] | None = None) -> list[dict]:
         """List Podman containers with optional filters."""
+        filters = filters or {}
         # Work-around for https://github.com/containers/podman-py/issues/542
         for k, v in filters.items():
             if len(v) == 1:

kubeflow/trainer/backends/container/adapters/podman_test.py

@@ -0,0 +1,73 @@
+# Copyright 2025 The Kubeflow Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Tests for PodmanClientAdapter."""
+
+from unittest.mock import Mock
+
+from kubeflow.trainer.backends.container.adapters.podman import PodmanClientAdapter
+
+
+def test_list_containers_without_filters():
+    """Test that list_containers() with no filters lists all containers."""
+    adapter = PodmanClientAdapter.__new__(PodmanClientAdapter)
+    container = Mock()
+    container.id = "abc123"
+    container.name = "job-0"
+    container.labels = {"app": "trainjob"}
+    container.status = "running"
+    container.attrs = {"Created": "2025-01-01T00:00:00Z"}
+    adapter.client = Mock()
+    adapter.client.containers.list.return_value = [container]
+
+    result = adapter.list_containers()
+
+    adapter.client.containers.list.assert_called_once_with(all=True, filters={})
+    assert result == [
+        {
+            "id": "abc123",
+            "name": "job-0",
+            "labels": {"app": "trainjob"},
+            "status": "running",
+            "created": "2025-01-01T00:00:00Z",
+        }
+    ]
+
+
+def test_list_containers_with_filters():
+    """Test that a single-value label filter is collapsed and forwarded to Podman."""
+    adapter = PodmanClientAdapter.__new__(PodmanClientAdapter)
+    container = Mock()
+    container.id = "abc123"
+    container.name = "job-0"
+    container.labels = {"app": "trainjob"}
+    container.status = "running"
+    container.attrs = {"Created": "2025-01-01T00:00:00Z"}
+    adapter.client = Mock()
+    adapter.client.containers.list.return_value = [container]
+
+    result = adapter.list_containers(filters={"label": ["app=trainjob"]})
+
+    adapter.client.containers.list.assert_called_once_with(
+        all=True, filters={"label": "app=trainjob"}
+    )
+    assert result == [
+        {
+            "id": "abc123",
+            "name": "job-0",
+            "labels": {"app": "trainjob"},
+            "status": "running",
+            "created": "2025-01-01T00:00:00Z",
+        }
+    ]