PEP 828: add synchonous delegation recipe

peps/pep-0828.rst

@@ -379,16 +379,47 @@ asynchronous generator methods: :meth:`~agen.asend` to :meth:`~generator.send`,
 :meth:`~agen.athrow` to :meth:`~generator.throw`, and :meth:`~agen.aclose`
 to :meth:`~generator.close`.
 
-For example, asynchronous exceptions could be injected into synchronous
-generators:
+It's trivial for anyone that needs to delegate to a subgenerator to write
+the wrapper class to upgrade a synchronous :class:`~collections.abc.Iterable`
+or :class:`~collections.abc.Generator` to an
+async one before calling ``async yield from``.
 
 .. code-block:: python
 
+    class AsAsyncIterator:
+        def __init__(self, wrapped):
+            self._wrapped = iter(wrapped)
+
+        def __aiter__(self):
+            return self
+
+        async def __anext__(self):
+            try:
+                return self._wrapped.__next__()
+            except StopIteration as e:
+                raise StopAsyncIteration(e.value) from e
+
+
+    class AsAsyncGenerator(AsAsyncIterator):
+        async def asend(self, value):
+            try:
+                return self._wrapped.send(value)
+            except StopIteration as e:
+                raise StopAsyncIteration(e.value) from e
+
+        async def athrow(self, exc):
+            try:
+                return self._wrapped.throw(exc)
+            except StopIteration as e:
+                raise StopAsyncIteration(e.value) from e
+
+        async def aclose(self):
+            return self._wrapped.close()
+
+
     async def agen():
-        async with asyncio.timeout(3):
-            # If the timeout fails, then an asyncio.TimeoutError would be raised
-            # in a *synchronous* generator!
-            yield from subgen()
+        async yield from AsAsyncIterator([1, 2, 3])
+        async yield from AsAsyncGenerator(subgen())
 
 
 To quote Brandt Bucher (paraphrased):