docs: using Iterators instead of Generators

README.md

@@ -179,6 +179,35 @@ Second, we pass parameters for task generation to the child proceses, not tasks.
 
 Third, as described in the narrative for Listing 5, the GreenTaskQueue has less context-switching overhead than ordinary multithreaded TaskQueue. Using GreenTaskQueue will cause each core to efficiently run independently of the others. At this point, your main bottlenecks will probably be OS/network card related (let us know if they aren't!). Multiprocessing does scale task production, but it's not 1:1 in the number of processes. The number of tasks per a process will fall with each additional core added, but each core still adds additional throughput up to about 16 cores.
 
+```python 
+# Listing 7: Exchanging Generators for Iterators
+import gevent.monkey 
+gevent.monkey.patch_all()
+from taskqueue import GreenTaskQueue 
+from concurrent.futures import ProcessPoolExecutor 
+
+class PrintTaskIterator(object):
+  def __init__(self, start, end):
+    self.start = start 
+    self.end = end 
+  def __len__(self):
+    return self.end - self.start
+  def __iter__(self):
+    for i in range(self.start, self.end):
+      yield PrintTask(i)
+
+def upload(tsks):
+  tq = GreenTaskQueue('wms-test-pull-queue-2')
+  tq.insert_all(tsks)
+
+tasks = [ PrintTaskIterator(0, 100), PrintTaskIterator(100, 200) ]
+with ProcessPoolExecutor(max_workers=2) as execute:
+  execute.map(upload, tasks)
+```
+
+If you insist on wanting to pass generators to your subprocesses, you can use iterators instead. The construction above allows us to write the generator call up front, pass only a few primatives through the pickling process, and transparently call the generator on the other side. We can even support the `len()` function which is not available for generators.
+
+
 [1] You can't pass generators in CPython but [you can pass iterators](https://stackoverflow.com/questions/1939015/singleton-python-generator-or-pickle-a-python-generator/1939493#1939493). You can pass generators if you use Pypy or Stackless Python.
 
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