Input validation for num_nodes argument (#18598)

src/lightning/fabric/connector.py

@@ -293,6 +293,9 @@ def _check_config_and_set_final_flags(
                 self._parallel_devices = self._strategy_flag.parallel_devices
 
     def _check_device_config_and_set_final_flags(self, devices: Union[List[int], str, int], num_nodes: int) -> None:
+        if not isinstance(num_nodes, int) or num_nodes < 1:
+            raise ValueError(f"`num_nodes` must be a positive integer, but got {num_nodes}.")
+
         self._num_nodes_flag = num_nodes
         self._devices_flag = devices
 

src/lightning/pytorch/trainer/connectors/accelerator_connector.py

@@ -317,11 +317,10 @@ def _check_config_and_set_final_flags(
                     self._accelerator_flag = "cuda"
                 self._parallel_devices = self._strategy_flag.parallel_devices
 
-    def _check_device_config_and_set_final_flags(
-        self,
-        devices: Union[List[int], str, int],
-        num_nodes: int,
-    ) -> None:
+    def _check_device_config_and_set_final_flags(self, devices: Union[List[int], str, int], num_nodes: int) -> None:
+        if not isinstance(num_nodes, int) or num_nodes < 1:
+            raise ValueError(f"`num_nodes` must be a positive integer, but got {num_nodes}.")
+
         self._num_nodes_flag = num_nodes
         self._devices_flag = devices
 

tests/tests_fabric/test_connector.py

@@ -314,6 +314,13 @@ def test_strategy_choice_multi_node_gpu(_, strategy, strategy_class, devices):
     assert isinstance(connector.strategy, strategy_class)
 
 
+def test_num_nodes_input_validation():
+    with pytest.raises(ValueError, match="`num_nodes` must be a positive integer"):
+        _Connector(num_nodes=0)
+    with pytest.raises(ValueError, match="`num_nodes` must be a positive integer"):
+        _Connector(num_nodes=-1)
+
+
 @mock.patch("lightning.fabric.accelerators.cuda.num_cuda_devices", return_value=0)
 def test_cuda_accelerator_can_not_run_on_system(_):
     connector = _Connector(accelerator="cpu")

tests/tests_pytorch/trainer/connectors/test_accelerator_connector.py

@@ -1013,6 +1013,13 @@ def test_connector_sets_num_nodes(strategy, cuda_count_2):
     assert trainer.strategy.num_nodes == 2
 
 
+def test_connector_num_nodes_input_validation():
+    with pytest.raises(ValueError, match="`num_nodes` must be a positive integer"):
+        _AcceleratorConnector(num_nodes=0)
+    with pytest.raises(ValueError, match="`num_nodes` must be a positive integer"):
+        _AcceleratorConnector(num_nodes=-1)
+
+
 @pytest.mark.parametrize(
     ("precision_str", "strategy_str", "expected_precision_cls"),
     [