Merge 3.5 (issue #27243)

Doc/reference/compound_stmts.rst

@@ -726,7 +726,7 @@ The following code::
 Is semantically equivalent to::
 
     iter = (ITER)
-    iter = await type(iter).__aiter__(iter)
+    iter = type(iter).__aiter__(iter)
     running = True
     while running:
         try:

Doc/reference/datamodel.rst

@@ -2360,6 +2360,7 @@ generators, coroutines do not directly support iteration.
    Coroutine objects are automatically closed using the above process when
    they are about to be destroyed.
 
+.. _async-iterators:
 
 Asynchronous Iterators
 ----------------------
@@ -2372,7 +2373,7 @@ Asynchronous iterators can be used in an :keyword:`async for` statement.
 
 .. method:: object.__aiter__(self)
 
-   Must return an *awaitable* resulting in an *asynchronous iterator* object.
+   Must return an *asynchronous iterator* object.
 
 .. method:: object.__anext__(self)
 
@@ -2385,7 +2386,7 @@ An example of an asynchronous iterable object::
         async def readline(self):
             ...
 
-        async def __aiter__(self):
+        def __aiter__(self):
             return self
 
         async def __anext__(self):
@@ -2396,6 +2397,49 @@ An example of an asynchronous iterable object::
 
 .. versionadded:: 3.5
 
+.. note::
+
+   .. versionchanged:: 3.5.2
+      Starting with CPython 3.5.2, ``__aiter__`` can directly return
+      :term:`asynchronous iterators <asynchronous iterator>`.  Returning
+      an :term:`awaitable` object will result in a
+      :exc:`PendingDeprecationWarning`.
+
+      The recommended way of writing backwards compatible code in
+      CPython 3.5.x is to continue returning awaitables from
+      ``__aiter__``.  If you want to avoid the PendingDeprecationWarning
+      and keep the code backwards compatible, the following decorator
+      can be used::
+
+          import functools
+          import sys
+
+          if sys.version_info < (3, 5, 2):
+              def aiter_compat(func):
+                  @functools.wraps(func)
+                  async def wrapper(self):
+                      return func(self)
+                  return wrapper
+          else:
+              def aiter_compat(func):
+                  return func
+
+      Example::
+
+          class AsyncIterator:
+
+              @aiter_compat
+              def __aiter__(self):
+                  return self
+
+              async def __anext__(self):
+                  ...
+
+      Starting with CPython 3.6, the :exc:`PendingDeprecationWarning`
+      will be replaced with the :exc:`DeprecationWarning`.
+      In CPython 3.7, returning an awaitable from ``__aiter__`` will
+      result in a :exc:`RuntimeError`.
+
 
 Asynchronous Context Managers
 -----------------------------