bpo-39529: Deprecate creating new event loop in asyncio.get_event_loop() (GH-23554)

asyncio.get_event_loop() emits now a deprecation warning when it creates a new event loop.
In future releases it will became an alias of asyncio.get_running_loop().

Lib/asyncio/tasks.py

@@ -549,7 +549,7 @@ def as_completed(fs, *, timeout=None):
     from .queues import Queue  # Import here to avoid circular import problem.
     done = Queue()
 
-    loop = events.get_event_loop()
+    loop = events._get_event_loop()
     todo = {ensure_future(f, loop=loop) for f in set(fs)}
     timeout_handle = None
 
@@ -616,23 +616,26 @@ def ensure_future(coro_or_future, *, loop=None):
 
     If the argument is a Future, it is returned directly.
     """
-    if coroutines.iscoroutine(coro_or_future):
-        if loop is None:
-            loop = events.get_event_loop()
-        task = loop.create_task(coro_or_future)
-        if task._source_traceback:
-            del task._source_traceback[-1]
-        return task
-    elif futures.isfuture(coro_or_future):
+    return _ensure_future(coro_or_future, loop=loop)
+
+
+def _ensure_future(coro_or_future, *, loop=None):
+    if futures.isfuture(coro_or_future):
         if loop is not None and loop is not futures._get_loop(coro_or_future):
             raise ValueError('The future belongs to a different loop than '
-                             'the one specified as the loop argument')
+                            'the one specified as the loop argument')
         return coro_or_future
-    elif inspect.isawaitable(coro_or_future):
-        return ensure_future(_wrap_awaitable(coro_or_future), loop=loop)
-    else:
-        raise TypeError('An asyncio.Future, a coroutine or an awaitable is '
-                        'required')
+
+    if not coroutines.iscoroutine(coro_or_future):
+        if inspect.isawaitable(coro_or_future):
+            coro_or_future = _wrap_awaitable(coro_or_future)
+        else:
+            raise TypeError('An asyncio.Future, a coroutine or an awaitable '
+                            'is required')
+
+    if loop is None:
+        loop = events._get_event_loop(stacklevel=4)
+    return loop.create_task(coro_or_future)
 
 
 @types.coroutine
@@ -655,7 +658,8 @@ class _GatheringFuture(futures.Future):
     cancelled.
     """
 
-    def __init__(self, children, *, loop=None):
+    def __init__(self, children, *, loop):
+        assert loop is not None
         super().__init__(loop=loop)
         self._children = children
         self._cancel_requested = False
@@ -706,7 +710,7 @@ def gather(*coros_or_futures, return_exceptions=False):
     gather won't cancel any other awaitables.
     """
     if not coros_or_futures:
-        loop = events.get_event_loop()
+        loop = events._get_event_loop()
         outer = loop.create_future()
         outer.set_result([])
         return outer
@@ -773,7 +777,7 @@ def gather(*coros_or_futures, return_exceptions=False):
     loop = None
     for arg in coros_or_futures:
         if arg not in arg_to_fut:
-            fut = ensure_future(arg, loop=loop)
+            fut = _ensure_future(arg, loop=loop)
             if loop is None:
                 loop = futures._get_loop(fut)
             if fut is not arg:
@@ -823,7 +827,7 @@ def shield(arg):
         except CancelledError:
             res = None
     """
-    inner = ensure_future(arg)
+    inner = _ensure_future(arg)
     if inner.done():
         # Shortcut.
         return inner