Fix exception when creating a multiprocessing Pool after importing Lightning

src/lightning_lite/strategies/launchers/subprocess_script.py

@@ -17,7 +17,6 @@
 from time import sleep
 from typing import Any, Callable, Sequence
 
-import __main__
 import numpy as np
 from lightning_utilities.core.imports import RequirementCache
 
@@ -142,13 +141,16 @@ def _check_can_spawn_children(self) -> None:
 
 
 def _basic_subprocess_cmd() -> Sequence[str]:
+    import __main__  # local import to avoid https://github.com/Lightning-AI/lightning/issues/15218
+
     if __main__.__spec__ is None:  # pragma: no-cover
         return [sys.executable, os.path.abspath(sys.argv[0])] + sys.argv[1:]
     else:
         return [sys.executable, "-m", __main__.__spec__.name] + sys.argv[1:]
 
 
 def _hydra_subprocess_cmd(local_rank: int) -> Sequence[str]:
+    import __main__  # local import to avoid https://github.com/Lightning-AI/lightning/issues/15218
     from hydra.core.hydra_config import HydraConfig
     from hydra.utils import to_absolute_path
 

src/pytorch_lightning/CHANGELOG.md

@@ -186,6 +186,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Fixed batch normalization statistics calculation in `StochasticWeightAveraging` callback ([#14866](https://github.com/Lightning-AI/lightning/pull/14866))
 - Avoided initializing optimizers during deepspeed inference ([#14944](https://github.com/Lightning-AI/lightning/pull/14944))
 - Fixed `LightningCLI` parse_env and description in subcommands ([#15138](https://github.com/Lightning-AI/lightning/pull/15138))
+- Fixed an exception that would occur when creating a `multiprocessing.Pool` after importing Lightning ([#15292](https://github.com/Lightning-AI/lightning/pull/15292))
 
 
 ## [1.7.7] - 2022-09-22