Explicitly note the problem with functions defined in __main__.

ericsnowcurrently · ericsnowcurrently · commit f150931a8151 · 2024-10-08T15:31:15.000-06:00
diff --git a/Doc/library/concurrent.futures.rst b/Doc/library/concurrent.futures.rst
@@ -251,6 +251,10 @@ objects can be executed and returned.
    like with :class:`ProcessPoolExecutor`.  Likewise, functions (and
    arguments) passed to :meth:`~Executor.submit` are pickled.
 
+   .. note::
+      functions defined in the ``__main__`` module cannot be pickled
+      and thus cannot be used.
+
    *shared* is an optional dict of objects shared by all interpreters
    in the pool.  The items are added to each interpreter's ``__main__``
    module.  Not all objects are shareable.  Those that are include
diff --git a/Lib/concurrent/futures/interpreter.py b/Lib/concurrent/futures/interpreter.py
@@ -53,7 +53,9 @@ def resolve_task(fn, args, kwargs):
                 # XXX Keep the compiled code object?
                 compile(data, '<string>', 'exec')
             else:
-                # XXX This does not work if fn comes from the __main__ module.
+                # Functions defined in the __main__ module can't be pickled,
+                # so they can't be used here (for now).  We could possibly
+                # borrow from multiprocessing to work around this.
                 data = pickle.dumps((fn, args, kwargs))
                 kind = 'function'
             return (data, kind)
