You Might Not Want Async

First version given at PyCon TW 2016.
Revised for PyCon APAC 2016 (Seoul).
Revised for PyCon JP 2016.

Asynchrony in Python had gathered much momentum recently, with interests from core developers, as evidenced by the introduction of `asyncio` in Python 3.4, and a great boom of related third-party projects following it. By utilising more functionalities from the underlying operating system, it is a great solution to many existing problems in Python applications, gaining practical concurrency without working around the well-known GIL (global interpreter lock) problem.

With all its advantages, asynchrony is, however, still a relatively new concept in Python, and as a result could be somewhat mistaken, even misunderstood by some people. One of these misconceptions, probably the most serious, is to mistake concurrency through asynchrony for parallelism. Although `asyncio` (and other similar solutions) lets multiple parts of your program executes without interfering each other, it does *not* allow them to run together—this is still impossible, at least in CPython, due to the continued existence of the GIL. This makes asynchrony suitable for only a certain, instead of all, kinds of problems. Evaluation is therefore required before a programmer can decide whether the asynchrony model is suitable for a particular application.

Furthermore, partly due to its relatively short existence, paradigms in asynchrony programming do not necessarily fit well with other parts of Python, including libraries, either built-in or third-party ones. Since only blocking libraries were available in most of Python’s history, many assumptions they made may not work well with async programs out-of-the-box. Adopting asynchrony, at least at the present time, will therefore introduce more technical debt to your program. These are all important aspects that require much consideration before you dive head-first into asynchrony.

Transcript

1. You Might Not Want Async
2. Quick Questions • Concurrency with Python • Threads • Multi-processing • Single-thread asynchrony • asyncio
3. ࣗݾ঺հ (PyCon JP Ver.) • ৉ ࢠሯ • ͫΐ͏ ͪʔͽΜ • @uranusjr
4. Me • Call me TP • Follow @uranusjr • https://uranusjr.com
7. http://macdown.uranusjr.com
8. www. .com
9. 10–11 June 2017 (Ծ) 7
12. meh
13. 2014 Python 3.4 OSDC.tw PyCon APAC March April May
15. Got me thinking
17. Synchronous
18. (Single-Threaded) Async
19. Sync vs. Async
20. Absolute magic
21. Before After
22. Module 5 - Doctor Faustus by Christopher Marlowe
23. Not For You • Infects the whole program • Async is not parallelism • Third party support
24. import sqlite3 def read_data(dbname): con = sqlite3.connect(dbname) cur = con.cursor() cur.execute('SELECT * FROM data OFFSET 0 LIMIT 1') data = cur.fetchone() cur.close() con.close() return data
25. import aioodbc async def read_data(dbname): con = await aioodbc.connect( dsn='Driver=SQLite;Database={}'.format(dbname), ) cur = await con.cursor() await cur.execute('SELECT * FROM data OFFSET 0 LIMIT 1') data = await cur.fetchone() await cur.close() await con.close() return data
26. from .db import read_data def main(): data = read_data('data.sqlite3') print(data) main()
27. import asyncio from .db import read_data async def main(): data = await read_data('db.sqlite3') print(data) loop = asyncio.get_event_loop() loop.run_until_complete(main()) loop.close()
28. U+1F937 SHRUG (Unicode 9.0)
29. RELAX PEOPLE I’M JUST GETTING STARTED
30. import asyncio from .db import read_data async def main(): data = read_data('db.sqlite3') print(data) loop = asyncio.get_event_loop() loop.run_until_complete(main()) loop.close()
31. (demo) $ python demo.py <coroutine object read_data at 0x10e2d1f10>
32. import asyncio from .db import read_data async def main(): data = read_data('db.sqlite3') print(data) loop = asyncio.get_event_loop() loop.run_until_complete(main()) loop.close()
33. import asyncio from .db import read_data async def main(): data = await read_data('db.sqlite3') print(data) loop = asyncio.get_event_loop() loop.run_until_complete(main()) loop.close()
35. I KNOW WRITE UNIT TESTS
36. GOOD LUCK WITH THAT
39. import asyncio import time async def do_something(): print('Before', time.monotonic()) await asyncio.sleep(2) print('After ', time.monotonic())  await do_something() Before 199208.190632853 After 199210.192092425
40. import unittest class MyTestCase(unittest.TestCase): async def test_do_something(self): await do_something() if __name__ == '__main__': unittest.main()
41. (demo) $ python tests.py . ---------------------------------------------------- Ran 1 test in 0.002s OK
42. (demo) $ python tests.py . ---------------------------------------------------- Ran 1 test in 0.002s OK
43. What Happened? • unittest does not know about asyncio • Coroutine methods are executed “normally” • Called, but not executed (awaited)
44. WHAT IF I TOLD YOU THIS IS GONNA BE TOUGH
45. import asyncio import functools def asynchronous(func): @functools.wraps(func) def asynchronous_inner(*args, **kwargs): loop = asyncio.new_event_loop() asyncio.set_event_loop(loop) try: loop.run_until_complete(func(*args, **kwargs)) finally: loop.close() return asynchronous_inner
46. import unittest class MyTestCase(unittest.TestCase): @asynchronous async def test_do_something(self): await do_something() if __name__ == '__main__': unittest.main()
47. (demo) $ python tests.py Before 51728.420457105 After 51730.424515145 . ---------------------------------------------------- Ran 1 test in 2.006s OK
48. Tips • Beware of warning output • Especially if you redirect • Add coverage report to testing code • Consider pip install asynctest
49. Or use pytest instead
50. import time async def test_do_something(): before = time.monotonic() await do_something() delta_t = time.monotonic() - before assert -0.01 < delta_t < 0.01
51. (demo) $ py.test tests.py ====================== test session starts ====================== platform darwin -- Python 3.5.1, pytest-2.9.1, py-1.4.31, pluggy-0.3.1 collected 0 items / 1 errors ============================ ERRORS ============================= ___________________ ERROR collecting tests.py____________________ > for i, x in enumerate(self.obj()): E TypeError: 'coroutine' object is not iterable python3.5/site-packages/_pytest/python.py:765: TypeError ==================== 1 error in 0.15 seconds ====================
53. import time import pytest @pytest.mark.asyncio async def test_do_something(capsys): before = time.monotonic() await do_something() delta_t = time.monotonic() - before assert -0.01 < delta_t < 0.01
57. asyncio • Boilerplate • Error-prone • Immature API (I think)
58. import requests def collect_contents(urls): contents = [] for url in urls: resp = requests.get(url) if resp.status_code != 200: continue content = resp.text contents.append(content) return contents
59. import aiohttp async def collect_contents(urls): contents = [] with aiohttp.ClientSession() as session: for url in urls: async with session.get(url) as rest: if resp.status != 200: continue content = await resp.text() contents.append(content) return contents
60. 㖶װ׌ַ׵ׁ
61. import aiohttp async def collect_contents(urls): contents = [] with aiohttp.ClientSession() as session: for url in urls: async with session.get(url) as rest: if resp.status != 200: continue content = await resp.text() contents.append(content) return contents
63. import asyncio import aiohttp async def collect_contents(urls): coroutines = [] with aiohttp.ClientSession() as session: for url in urls: async with session.get(url) as rest: if resp.status != 200: continue coroutines.append(resp.text()) contents = await asyncio.gather(*coroutines) return contents
67. LET’S TRY SOMETHING “DIFFERENT”
68. Alternatives • concurrent & multiprocessing • Greenlets • Similar idea, but less infectious • C extension • threading • Standard I/O
69. http://greenlet.readthedocs.io
70. GEVENT MEH
71. ROUTINES ROUTINES EVERYWHERE
72. package main import ("fmt"; "time") func doSomething() { time.Sleep(2 * time.Second) fmt.Println(time.Now(), "Slept") } func main() { doSomething() fmt.Println(time.Now(), "OK") } 00:00:00 Slept 00:00:00 OK
73. package main import ("fmt"; "time") func doSomething() { time.Sleep(2 * time.Second) fmt.Println(time.Now(), "Slept") } func main() { go doSomething() fmt.Println(time.Now(), "OK") } 00:00:00 OK
74. package main import ("fmt"; "time") var sem = make(chan bool) func doSomething() { time.Sleep(2 * time.Second) fmt.Println(time.Now(), "Slept") sem  true } func main() { go doSomething() fmt.Println(time.Now(), "OK") sem } 00:00:00 OK 00:00:02 Slept
75. The Go Model • Boilerplate • Error-prone • Immature API (I think)
76. """Do something. Synchronous version.""" import datetime import time def do_something(): time.sleep(2) print(datetime.datetime.now(), 'Slept') do_something() print(datetime.datetime.now(), 'Done')
77. """Do something. Asynchronous version.""" import asyncio import datetime async def do_something(): await asyncio.sleep(2) print(datetime.datetime.now(), 'Slept') loop = asyncio.get_event_loop() task = loop.create_task(do_something()) print(datetime.datetime.now(), 'Done') loop.run_until_complete(asyncio.wait([task])) loop.close()
79. """Do something. Synchronous version.""" import datetime import time def do_something(): time.sleep(2) print(datetime.datetime.now(), 'Slept') do_something() print(datetime.datetime.now(), 'Done')
80. """What if I can just write this?""" import asyncio import datetime import time async def do_something(): await time.sleep(2) print(datetime.datetime.now(), 'Slept') await do_something() print(datetime.datetime.now(), 'Done') asyncio.run_event_loop()
81. I know, it’s not really possible.
82. At least we can dream. Or wait until Python 6.0?
83. Recap • Rant • Moar rant • Susceptible advice • Unrealistic dream
84. But Seriously • Asynchrony is not the silver bullet • It makes you jump through loops • There are alternatives • Fingers crossed