Task on main message

docs/cookbook.md

@@ -926,20 +926,29 @@ if __name__ == '__main__':
 For details, see [Safe importing of main module](https://docs.python.org/3/library/multiprocessing.html#multiprocessing-safe-main-import).
 
 
-### Why do I see the following error: `AttributeError: Can't get attribute 'YOUR_TASK_CLASS' on <module '__main__' (built-in)>`?
+<div id="spawn-interactive-main"></div>
 
-You will see this error (as part of a very long stack trace) when
-running a Lab with `runner_backend='spawn'` (the default on macOS and
-Windows) from an interactive Python shell.
+### Why do I see the following error: `RunnerError: Unable to submit YourTaskType tasks to SpawnProcessRunner because the task type is defined in the __main__ module from an interactive Python session`?
 
-The solution to this error is to define all of your labtech `Task`
-types in a separate `.py` Python module file which you can import into
-your interactive shell session (e.g. `from my_module import MyTask`).
+You may see this error when running a Lab with
+`runner_backend='spawn'` (the default on macOS and Windows) from an
+interactive Python shell (e.g. a Jupyter notebook session or a Python
+script).
+
+The solution to this error is to define all of the classes you are
+using from your labtech context and tasks (including task types) in a
+separate `.py` Python module file which you can import into your
+interactive shell session (e.g. `from my_module import MyClass`).
 
 The reason for this error is that "spawned" task subprocesses will not
 receive a copy the current state of your `__main__` module (which
 contains the variables you declare interactively in the Python shell,
-including task definitions). This error does not occur with
+including class definitions). This error does not occur with
 `runner_backend='fork'` (the default on Linux) because forked
 subprocesses *do* receive the current state of all modules (including
 `__main__`) from the parent process.
+
+
+### Why do I see the following error: `AttributeError: Can't get attribute 'YOUR_CLASS' on <module '__main__' (built-in)>`?
+
+[See the answer to the question directly above.](#spawn-interactive-main)

labtech/runners/process.py

@@ -22,7 +22,7 @@
 from labtech.monitor import get_process_info
 from labtech.tasks import get_direct_dependencies
 from labtech.types import Runner, RunnerBackend
-from labtech.utils import LoggerFileProxy, get_supported_start_methods, logger
+from labtech.utils import LoggerFileProxy, get_supported_start_methods, is_interactive, logger
 
 from .base import run_or_load_task
 
@@ -151,7 +151,7 @@ def _start_processes(self):
         for future in futures_to_start:
             thunk = self._pending_future_to_thunk[future]
             del self._pending_future_to_thunk[future]
-            process = multiprocessing.Process(
+            process = self.mp_context.Process(
                 target=_subprocess_target,
                 kwargs=dict(
                     future_id=future.id,
@@ -461,6 +461,16 @@ def _get_mp_context(self) -> SpawnContext:
 
     def _submit_task(self, executor: ProcessExecutor, task: Task, task_name: str,
                      use_cache: bool, process_event_queue: Queue, log_queue: Queue) -> Future:
+        if is_interactive() and task.__class__.__module__ == '__main__':
+            raise RunnerError(
+                (f'Unable to submit {task.__class__.__qualname__} tasks to '
+                 'SpawnProcessRunner because the task type is defined in the '
+                 '__main__ module from an interactive Python session. '
+                 'Please define your task types in a separate `.py` Python '
+                 'module file. For details, see: '
+                 'https://ben-denham.github.io/labtech/cookbook/#spawn-interactive-main')
+            )
+
         filtered_context: LabContext = {}
         results_map: dict[Task, TaskResult] = {}
         if not use_cache:

labtech/utils.py

@@ -5,6 +5,7 @@
 import logging
 import platform
 import re
+import sys
 from multiprocessing import get_all_start_methods
 from typing import TYPE_CHECKING, Generic, TypeVar, cast
 
@@ -123,6 +124,10 @@ def ensure_dict_key_str(value, *, exception_type: type[Exception]) -> str:
     return cast('str', value)
 
 
+def is_interactive() -> bool:
+    return hasattr(sys, 'ps1')
+
+
 def is_ipython() -> bool:
     return hasattr(builtins, '__IPYTHON__')
 
@@ -151,6 +156,7 @@ class tqdm_notebook(base_tqdm_notebook):
     'OrderedSet',
     'LoggerFileProxy',
     'ensure_dict_key_str',
+    'is_interactive',
     'is_ipython',
     'tqdm',
     'tqdm_notebook',