uasyncio: add test_can.py.

uasyncio_iostream/README.md

@@ -59,6 +59,8 @@ Hopefully these are self-documenting on import.
  I/O scheduling. Runs on Pyboard.
  3. `ms_timer.py` and `ms_timer_test.py` A practical use of priority scheduling to
  implement a timer with higher precision than `asyncio.sleep_ms`. Runs on Pyboard.
+ 4. `test_can.py` Demonstrates differences in behaviour between CPython 3.8 and
+ MicroPython. See code comments.
 
 # 5. CPython compatibility of user primitives
 

uasyncio_iostream/test_can.py

@@ -0,0 +1,62 @@
+# Shows that MicroPython seems to cancel a task earlier than CPython
+# Also demonstrates that CPython cancels tasks when run() terminates.
+try:
+    import asyncio
+except ImportError:
+    import uasyncio as asyncio
+
+async def foo(n):
+    try:
+        while True:
+            await asyncio.sleep(0)
+            print(n)
+    except asyncio.CancelledError:
+        print('Task {} canned.'.format(n))
+        raise
+
+async def main(n):
+    tasks = []
+    for n in range(3):
+        tasks.append(asyncio.create_task(foo(n)))
+    for _ in range(n):
+        await asyncio.sleep(0)
+    print('Cancelling task 1')
+    tasks[1].cancel()
+    for _ in range(3):
+        await asyncio.sleep(0)
+
+asyncio.run(main(n=3))
+
+# CPython 3.8
+#>>> import test_can
+#0
+#1
+#2
+#Cancelling task 1
+#0
+#Task 1 canned.
+#2
+#0
+#2
+#0
+#2
+#0
+#2
+#Task 0 canned.
+#Task 2 canned.
+#>>> 
+
+# MicroPython
+#>>> import test_can
+#0
+#1
+#2
+#Cancelling task 1
+#Task 1 canned.
+#0
+#2
+#0
+#2
+#0
+#2
+#>>>