final concurrency examples

futures/future_yield.py

@@ -0,0 +1,41 @@
+"""
+An experiment showing that ``asyncio.Future`` is an iterable (it
+implements `__iter__`) designed to be used with ``yield from``.
+
+Priming the future returns itself. After the result of the future
+is set, next iteration produces the result as the ``value`` attribute
+of ``StopIteration``.
+
+Sample run::
+
+    $ python3 future_yield.py
+    a, future:      <Future pending> 0x66514c
+    b, prime_res:   <Future pending> 0x66514c
+    b, exc.value:   42
+
+"""
+
+import asyncio
+
+@asyncio.coroutine
+def a(future):
+    print('a, future:\t', future, hex(id(future)))
+    res = yield from future
+    return res
+
+def b():
+    future = asyncio.Future()
+    coro = a(future)
+    prime_res = next(coro)
+    print('b, prime_res:\t', prime_res, hex(id(future)))
+    # If next(coro) is called again before the result of
+    # the future is set, we get:
+    #   AssertionError: yield from wasn't used with future
+    #result = next(coro)  # uncomment to see AssertionError
+    future.set_result(42)
+    try:
+        next(coro)
+    except StopIteration as exc:
+        print('b, exc.value:\t', exc.value)
+
+b()