Ensure accelerator is valid if running interactively

pytorch_lightning/trainer/connectors/accelerator_connector.py

@@ -13,6 +13,7 @@
 # limitations under the License.
 
 import os
+import sys
 from typing import List, Optional, Sequence, Union
 
 import torch
@@ -84,6 +85,9 @@ def __init__(
         # initialization
         self._device_type = DeviceType.CPU
         self._distrib_type = None
+        self.ipython_compatible_distrib_types = [
+            DistributedType.DP, DistributedType.DDP_SPAWN, DistributedType.DDP_SHARDED_SPAWN
+        ]
 
         self.num_processes = num_processes
         self.tpu_cores = device_parser.parse_tpu_cores(tpu_cores)
@@ -515,6 +519,9 @@ def set_distributed_mode(self, distributed_backend: Optional[str] = None):
                 rank_zero_warn('You are running on single node with no parallelization, so distributed has no effect.')
                 self._distrib_type = None
 
+        # finished configuring self._distrib_type, check ipython environment
+        self.check_ipython_compatibility()
+
         # for DDP overwrite nb processes by requested GPUs
         if (
             self._device_type == DeviceType.GPU
@@ -556,6 +563,23 @@ def _set_horovod_backend(self):
         else:
             self.num_processes = hvd.local_size()
 
+    def check_ipython_compatibility(self):
+        """
+        Raises a `MisconfigurationException` if the accelerator and/or plugin is not compatible with IPython
+        and code is run in an IPython kernel.
+        """
+        if self._distrib_type in self.ipython_compatible_distrib_types:
+            return
+        # check ipython env
+        if "IPython" in sys.modules:
+            from IPython import get_ipython
+            if get_ipython() is not None:
+                raise MisconfigurationException(
+                    f"Selected distributed backend {self._distrib_type} is not compatible with IPython environment."
+                    f" Run your code as a script, or choose one of the compatible backends:"
+                    f" {', '.join(self.ipython_compatible_distrib_types)}"
+                )
+
     def check_horovod(self):
         """Raises a `MisconfigurationException` if the Trainer is not configured correctly for Horovod."""
         if not _HOROVOD_AVAILABLE:

tests/accelerators/test_accelerator_connector.py

@@ -13,7 +13,9 @@
 # limitations under the License
 
 import os
+import sys
 from unittest import mock
+from unittest.mock import Mock
 
 import pytest
 import torch
@@ -25,6 +27,7 @@
 from pytorch_lightning.callbacks import Callback
 from pytorch_lightning.plugins import DDP2Plugin, DDPPlugin, DDPSpawnPlugin, PrecisionPlugin, SingleDevicePlugin
 from pytorch_lightning.plugins.environments import ClusterEnvironment, SLURMEnvironment, TorchElasticEnvironment
+from pytorch_lightning.utilities.exceptions import MisconfigurationException
 from tests.helpers.boring_model import BoringModel
 
 
@@ -378,3 +381,17 @@ def on_fit_start(self, trainer, pl_module):
 
     with pytest.raises(SystemExit):
         trainer.fit(model)
+
+
+@mock.patch.dict(sys.modules, {"IPython": Mock()})
+@mock.patch("IPython.get_ipython")
+@mock.patch('torch.cuda.device_count', return_value=2)
+def test_ipython_incompatible_backend_error(device_count_mock, ipython_mock):
+    with pytest.raises(MisconfigurationException, match="backend ddp is not compatible"):
+        Trainer(accelerator="ddp", gpus=2)
+
+    with pytest.raises(MisconfigurationException, match="backend ddp is not compatible"):
+        Trainer(accelerator="ddp_cpu", num_processes=2)
+
+    with pytest.raises(MisconfigurationException, match="backend ddp2 is not compatible"):
+        Trainer(accelerator="ddp2", gpus=2)