updated more ch. 18 examples to Python 3.7

18-asyncio-py3.7/countdown.py

@@ -0,0 +1,30 @@
+#!/usr/bin/env python3
+
+# Inspired by
+# https://snarky.ca/how-the-heck-does-async-await-work-in-python-3-5/
+
+import asyncio
+import time
+
+
+async def countdown(label, delay):
+    tabs = (ord(label) - ord('A')) * '\t'
+    n = 3
+    while n > 0:
+        await asyncio.sleep(delay)  # <----
+        dt = time.perf_counter() - t0
+        print('━' * 50)
+        print(f'{dt:7.4f}s \t{tabs}{label} = {n}')
+        n -= 1
+
+loop = asyncio.get_event_loop()
+tasks = [
+    loop.create_task(countdown('A', .7)),
+    loop.create_task(countdown('B', 2)),
+    loop.create_task(countdown('C', .3)),
+    loop.create_task(countdown('D', 1)),
+]
+t0 = time.perf_counter()
+loop.run_until_complete(asyncio.wait(tasks))
+loop.close()
+print('━' * 50)