Implement a none backend as default. (#94)

Author: tobiasraabe

src/pytask_parallel/backends.py

@@ -84,6 +84,8 @@ def _get_thread_pool_executor(n_workers: int) -> Executor:
 class ParallelBackend(Enum):
     """Choices for parallel backends."""
 
+    NONE = "none"
+
     CUSTOM = "custom"
     DASK = "dask"
     LOKY = "loky"

src/pytask_parallel/build.py

@@ -26,7 +26,7 @@ def pytask_extend_command_line_interface(cli: click.Group) -> None:
             ["--parallel-backend"],
             type=EnumChoice(ParallelBackend),
             help="Backend for the parallelization.",
-            default=ParallelBackend.LOKY,
+            default=ParallelBackend.NONE,
         ),
     ]
     cli.commands["build"].params.extend(additional_parameters)

src/pytask_parallel/config.py

@@ -21,9 +21,6 @@ def pytask_parse_config(config: dict[str, Any]) -> None:
     """Parse the configuration."""
     __tracebackhide__ = True
 
-    if config["n_workers"] == "auto":
-        config["n_workers"] = max(os.cpu_count() - 1, 1)
-
     try:
         config["parallel_backend"] = ParallelBackend(config["parallel_backend"])
     except ValueError:
@@ -33,6 +30,13 @@ def pytask_parse_config(config: dict[str, Any]) -> None:
         )
         raise ValueError(msg) from None
 
+    if config["n_workers"] == "auto":
+        config["n_workers"] = max(os.cpu_count() - 1, 1)
+
+    # If more than one worker is used, and no backend is set, use loky.
+    if config["n_workers"] > 1 and config["parallel_backend"] == ParallelBackend.NONE:
+        config["parallel_backend"] = ParallelBackend.LOKY
+
     config["delay"] = 0.1
 
 
@@ -43,6 +47,9 @@ def pytask_post_parse(config: dict[str, Any]) -> None:
     if config["pdb"] or config["trace"] or config["dry_run"]:
         return
 
+    if config["parallel_backend"] == ParallelBackend.NONE:
+        return
+
     # Register parallel execute and logging hook.
     config["pm"].register(logging)
     config["pm"].register(execute)