Add fan-out / fan-in Python snippet

Signed-off-by: Chris Gillum <cgillum@microsoft.com>

daprdocs/content/en/developing-applications/building-blocks/workflow/workflow-patterns.md

@@ -166,7 +166,38 @@ await context.CallActivityAsync("PostResults", sum);
 <!--python-->
 
 ```python
-# TODO
+import time
+from typing import List
+import dapr.ext.workflow as wf
+
+
+def batch_processing_workflow(ctx: wf.DaprWorkflowContext, wf_input: int):
+    # get a batch of N work items to process in parallel
+    work_batch = yield ctx.call_activity(get_work_batch, input=wf_input)
+
+    # schedule N parallel tasks to process the work items and wait for all to complete
+    parallel_tasks = [ctx.call_activity(process_work_item, input=work_item) for work_item in work_batch]
+    outputs = yield wf.when_all(parallel_tasks)
+
+    # aggregate the results and send them to another activity
+    total = sum(outputs)
+    yield ctx.call_activity(process_results, input=total)
+
+
+def get_work_batch(ctx, batch_size: int) -> List[int]:
+    return [i + 1 for i in range(batch_size)]
+
+
+def process_work_item(ctx, work_item: int) -> int:
+    print(f'Processing work item: {work_item}.')
+    time.sleep(5)
+    result = work_item * 2
+    print(f'Work item {work_item} processed. Result: {result}.')
+    return result
+
+
+def process_results(ctx, final_result: int):
+    print(f'Final result: {final_result}.')
 ```
 
 {{% /codetab %}}