Support a `._state.last_actor()` getter

Not sure if it's really that useful other then for reporting errors from
`current_actor()` but at least it alerts `tractor` devs and/or users
when the runtime has already terminated vs. hasn't been started
yet/correctly.

Set the `._last_actor_terminated: tuple` in the root's final block which
allows testing for an already terminated tree which is the case where
`._state._current_actor == None` and the last is set.

tractor/_root.py

@@ -262,6 +262,7 @@ async def open_root_actor(
                 await actor.cancel(None)  # self cancel
     finally:
         _state._current_actor = None
+        _state._last_actor_terminated = actor
 
         # restore breakpoint hook state
         sys.breakpointhook = builtin_bp_handler

tractor/_state.py

@@ -18,12 +18,18 @@
 Per process state
 
 """
+from __future__ import annotations
 from typing import (
-    Optional,
     Any,
+    TYPE_CHECKING,
 )
 
-_current_actor: Optional['Actor'] = None  # type: ignore # noqa
+if TYPE_CHECKING:
+    from ._runtime import Actor
+
+
+_current_actor: Actor|None = None  # type: ignore # noqa
+_last_actor_terminated: Actor|None = None
 _runtime_vars: dict[str, Any] = {
     '_debug_mode': False,
     '_is_root': False,
@@ -31,14 +37,49 @@ _runtime_vars: dict[str, Any] = {
 }
 
 
-def current_actor(err_on_no_runtime: bool = True) -> 'Actor':  # type: ignore # noqa
+def last_actor() -> Actor|None:
+    '''
+    Try to return last active `Actor` singleton
+    for this process.
+
+    For case where runtime already exited but someone is asking
+    about the "last" actor probably to get its `.uid: tuple`.
+
+    '''
+    return _last_actor_terminated
+
+
+def current_actor(
+    err_on_no_runtime: bool = True,
+) -> Actor:
     '''
     Get the process-local actor instance.
 
     '''
-    from ._exceptions import NoRuntime
-    if _current_actor is None and err_on_no_runtime:
-        raise NoRuntime("No local actor has been initialized yet")
+    if (
+        err_on_no_runtime
+        and _current_actor is None
+    ):
+        msg: str = 'No local actor has been initialized yet'
+        from ._exceptions import NoRuntime
+
+        if last := last_actor():
+            msg += (
+                f'Apparently the lact active actor was\n'
+                f'|_{last}\n'
+                f'|_{last.uid}\n'
+            )
+        # no actor runtime has (as of yet) ever been started for
+        # this process.
+        else:
+            msg += (
+                'No last actor found?\n'
+                'Did you forget to open one of:\n\n'
+                '- `tractor.open_root_actor()`\n'
+                '- `tractor.open_nursery()`\n'
+            )
+
+        raise NoRuntime(msg)
 
     return _current_actor
 

tractor/_supervise.py

@@ -534,12 +534,15 @@ async def open_nursery(
 
     '''
     implicit_runtime: bool = False
-
-    actor = current_actor(err_on_no_runtime=False)
+    actor: Actor = current_actor(
+        err_on_no_runtime=False
+    )
 
     try:
-        if actor is None and is_main_process():
-
+        if (
+            actor is None
+            and is_main_process()
+        ):
             # if we are the parent process start the
             # actor runtime implicitly
             log.info("Starting actor runtime!")