bpo-32643: Drop support for a few private Task and Future APIs. (#5293)

Specifically, it's not possible to subclass Task/Future classes
and override the following methods:

* Future._schedule_callbacks
* Task._step
* Task._wakeup

Author: 1st1

Lib/asyncio/futures.py

@@ -131,10 +131,10 @@ def cancel(self):
         if self._state != _PENDING:
             return False
         self._state = _CANCELLED
-        self._schedule_callbacks()
+        self.__schedule_callbacks()
         return True
 
-    def _schedule_callbacks(self):
+    def __schedule_callbacks(self):
         """Internal: Ask the event loop to call all callbacks.
 
         The callbacks are scheduled to be called as soon as possible. Also
@@ -234,7 +234,7 @@ def set_result(self, result):
             raise InvalidStateError('{}: {!r}'.format(self._state, self))
         self._result = result
         self._state = _FINISHED
-        self._schedule_callbacks()
+        self.__schedule_callbacks()
 
     def set_exception(self, exception):
         """Mark the future done and set an exception.
@@ -251,7 +251,7 @@ def set_exception(self, exception):
                             "and cannot be raised into a Future")
         self._exception = exception
         self._state = _FINISHED
-        self._schedule_callbacks()
+        self.__schedule_callbacks()
         self.__log_traceback = True
 
     def __await__(self):

Lib/asyncio/tasks.py

@@ -99,7 +99,7 @@ def __init__(self, coro, *, loop=None):
         self._coro = coro
         self._context = contextvars.copy_context()
 
-        self._loop.call_soon(self._step, context=self._context)
+        self._loop.call_soon(self.__step, context=self._context)
         _register_task(self)
 
     def __del__(self):
@@ -185,11 +185,11 @@ def cancel(self):
                 # catches and ignores the cancellation so we may have
                 # to cancel it again later.
                 return True
-        # It must be the case that self._step is already scheduled.
+        # It must be the case that self.__step is already scheduled.
         self._must_cancel = True
         return True
 
-    def _step(self, exc=None):
+    def __step(self, exc=None):
         if self.done():
             raise futures.InvalidStateError(
                 f'_step(): already done: {self!r}, {exc!r}')
@@ -232,17 +232,17 @@ def _step(self, exc=None):
                         f'Task {self!r} got Future '
                         f'{result!r} attached to a different loop')
                     self._loop.call_soon(
-                        self._step, new_exc, context=self._context)
+                        self.__step, new_exc, context=self._context)
                 elif blocking:
                     if result is self:
                         new_exc = RuntimeError(
                             f'Task cannot await on itself: {self!r}')
                         self._loop.call_soon(
-                            self._step, new_exc, context=self._context)
+                            self.__step, new_exc, context=self._context)
                     else:
                         result._asyncio_future_blocking = False
                         result.add_done_callback(
-                            self._wakeup, context=self._context)
+                            self.__wakeup, context=self._context)
                         self._fut_waiter = result
                         if self._must_cancel:
                             if self._fut_waiter.cancel():
@@ -252,41 +252,41 @@ def _step(self, exc=None):
                         f'yield was used instead of yield from '
                         f'in task {self!r} with {result!r}')
                     self._loop.call_soon(
-                        self._step, new_exc, context=self._context)
+                        self.__step, new_exc, context=self._context)
 
             elif result is None:
                 # Bare yield relinquishes control for one event loop iteration.
-                self._loop.call_soon(self._step, context=self._context)
+                self._loop.call_soon(self.__step, context=self._context)
             elif inspect.isgenerator(result):
                 # Yielding a generator is just wrong.
                 new_exc = RuntimeError(
                     f'yield was used instead of yield from for '
                     f'generator in task {self!r} with {result}')
                 self._loop.call_soon(
-                    self._step, new_exc, context=self._context)
+                    self.__step, new_exc, context=self._context)
             else:
                 # Yielding something else is an error.
                 new_exc = RuntimeError(f'Task got bad yield: {result!r}')
                 self._loop.call_soon(
-                    self._step, new_exc, context=self._context)
+                    self.__step, new_exc, context=self._context)
         finally:
             _leave_task(self._loop, self)
             self = None  # Needed to break cycles when an exception occurs.
 
-    def _wakeup(self, future):
+    def __wakeup(self, future):
         try:
             future.result()
         except Exception as exc:
             # This may also be a cancellation.
-            self._step(exc)
+            self.__step(exc)
         else:
             # Don't pass the value of `future.result()` explicitly,
             # as `Future.__iter__` and `Future.__await__` don't need it.
             # If we call `_step(value, None)` instead of `_step()`,
             # Python eval loop would use `.send(value)` method call,
             # instead of `__next__()`, which is slower for futures
             # that return non-generator iterators from their `__iter__`.
-            self._step()
+            self.__step()
         self = None  # Needed to break cycles when an exception occurs.
 
 
@@ -513,7 +513,7 @@ def __sleep0():
 
     This is a private helper for 'asyncio.sleep()', used
     when the 'delay' is set to 0.  It uses a bare 'yield'
-    expression (which Task._step knows how to handle)
+    expression (which Task.__step knows how to handle)
     instead of creating a Future object.
     """
     yield

Lib/test/test_asyncio/test_futures.py

@@ -175,10 +175,6 @@ def test_uninitialized(self):
         with self.assertRaises((RuntimeError, AttributeError)):
             fut.remove_done_callback(lambda f: None)
 
-        fut = self.cls.__new__(self.cls, loop=self.loop)
-        with self.assertRaises((RuntimeError, AttributeError)):
-            fut._schedule_callbacks()
-
         fut = self.cls.__new__(self.cls, loop=self.loop)
         try:
             repr(fut)

Lib/test/test_asyncio/test_tasks.py

@@ -1401,17 +1401,6 @@ def wait_for_future():
         self.assertTrue(t.done())
         self.assertIsNone(t.result())
 
-    def test_step_with_baseexception(self):
-        @asyncio.coroutine
-        def notmutch():
-            raise BaseException()
-
-        task = self.new_task(self.loop, notmutch())
-        self.assertRaises(BaseException, task._step)
-
-        self.assertTrue(task.done())
-        self.assertIsInstance(task.exception(), BaseException)
-
     def test_baseexception_during_cancel(self):
 
         def gen():
@@ -2275,22 +2264,12 @@ def __init__(self, *args, **kwargs):
             self.calls = collections.defaultdict(lambda: 0)
             super().__init__(*args, **kwargs)
 
-        def _schedule_callbacks(self):
-            self.calls['_schedule_callbacks'] += 1
-            return super()._schedule_callbacks()
-
         def add_done_callback(self, *args, **kwargs):
             self.calls['add_done_callback'] += 1
             return super().add_done_callback(*args, **kwargs)
 
     class Task(CommonFuture, BaseTask):
-        def _step(self, *args):
-            self.calls['_step'] += 1
-            return super()._step(*args)
-
-        def _wakeup(self, *args):
-            self.calls['_wakeup'] += 1
-            return super()._wakeup(*args)
+        pass
 
     class Future(CommonFuture, BaseFuture):
         pass
@@ -2310,12 +2289,11 @@ async def func():
 
         self.assertEqual(
             dict(task.calls),
-            {'_step': 2, '_wakeup': 1, 'add_done_callback': 1,
-             '_schedule_callbacks': 1})
+            {'add_done_callback': 1})
 
         self.assertEqual(
             dict(fut.calls),
-            {'add_done_callback': 1, '_schedule_callbacks': 1})
+            {'add_done_callback': 1})
 
     # Add patched Task & Future back to the test case
     cls.Task = Task

Misc/NEWS.d/next/Library/2018-01-24-00-32-58.bpo-32643.VWipsW.rst

@@ -0,0 +1,2 @@
+Make Task._step, Task._wakeup and Future._schedule_callbacks methods
+private.