Rename Future._blocking to _asyncio_future_blocking.

This is now an official "protected" API that can be used to write
classes that are duck-type-compatible with Future without subclassing
it.  (For that purpose I also changed isinstance(result, Future) to
check for this attribute instead.)

Hopefully Amber Brown can use this to make Twisted.Deferred compatible
with asyncio.Future.

Tests and docs are TBD.

1 files changed, 10 insertions, 2 deletions

diff --git a/Lib/asyncio/futures.py b/Lib/asyncio/futures.py
index 1feba4d370..edc13dcc47 100644
--- a/Lib/asyncio/futures.py
+++ b/Lib/asyncio/futures.py
@@ -134,7 +134,15 @@ class Future:
     _loop = None
     _source_traceback = None
 
-    _blocking = False  # proper use of future (yield vs yield from)
+    # This field is used for a dual purpose:
+    # - Its presence is a marker to declare that a class implements
+    #   the Future protocol (i.e. is intended to be duck-type compatible).
+    #   The value must also be not-None, to enable a subclass to declare
+    #   that it is not compatible by setting this to None.
+    # - It is set by __iter__() below so that Task._step() can tell
+    #   the difference between `yield from Future()` (correct) vs.
+    #   `yield Future()` (incorrect).
+    _asyncio_future_blocking = False
 
     _log_traceback = False   # Used for Python 3.4 and later
     _tb_logger = None        # Used for Python 3.3 only
@@ -357,7 +365,7 @@ class Future:
 
     def __iter__(self):
         if not self.done():
-            self._blocking = True
+            self._asyncio_future_blocking = True
             yield self  # This tells Task to wait for completion.
         assert self.done(), "yield from wasn't used with future"
         return self.result()  # May raise too.