ch20 examples

20-concurrency/spinner_async_experiment.py

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+# spinner_async_experiment.py
+
+# credits: Example by Luciano Ramalho inspired by
+# Michele Simionato's multiprocessing example in the python-list:
+# https://mail.python.org/pipermail/python-list/2009-February/675659.html
+
+import asyncio
+import itertools
+import time
+
+async def spin(msg: str) -> None:
+    for char in itertools.cycle(r'\|/-'):
+        status = f'\r{char} {msg}'
+        print(status, flush=True, end='')
+        try:
+            await asyncio.sleep(.1)
+        except asyncio.CancelledError:
+            break
+    print('THIS WILL NEVER BE OUTPUT')
+
+# tag::SPINNER_ASYNC_EXPERIMENT[]
+async def slow() -> int:
+    time.sleep(3)  # <4>
+    return 42
+
+async def supervisor() -> int:
+    spinner = asyncio.create_task(spin('thinking!'))  # <1>
+    print('spinner object:', spinner)  # <2>
+    result = await slow()  # <3>
+    spinner.cancel()  # <5>
+    return result
+# end::SPINNER_ASYNC_EXPERIMENT[]
+
+def main() -> None:
+    result = asyncio.run(supervisor())
+    print('Answer:', result)
+
+if __name__ == '__main__':
+    main()