TRIO STRUCTURED CONCURRENCY FOR PYTHON Jeremy Thurgood PyConZA 2020 1 

A LITTLE HISTORY PRELUDE 2 

THE ERA OF GOTO 10 I = 1 20 PRINT "LINE NUMBER"; I 30 I = I + 1 40 IF I <= 10 GOTO 20 50 PRINT "ALL DONE" 3 

MORE COMPLICATED GOTO 10 INPUT "HOW MANY PRIMES"; N : DIM P(N-1) 20 PRINT 2 : PRINT 3 30 P(1) = 3 : PI = 1 40 C = P(PI) 50 C = C+2 : I = 0 60 I = I+1 70 IF C/P(I) = INT(C/P(I)) GOTO 50 80 IF I < PI GOTO 60 90 PRINT C : PI = PI+1 : P(PI) = C 100 IF PI < N-2 GOTO 40 4 

STRUCTURED PROGRAMMING Ten lines: for i in range(10): print("LINE NUMBER", i+1) print("ALL DONE") 1 2 3 Prime Numbers: n = int(input("HOW MANY PRIMES? ")) print(2) primes = [] candidate = 3 while len(primes) < n-1: for p in primes: if candidate % p == 0: break else: # Runs if the loop ends without breaking. primes.append(candidate) print(candidate) candidate += 2 1 2 3 4 5 6 7 8 9 10 11 12 5 

CONSEQUENCES OF STRUCTURE Execution ow that humans can reason about Call stacks and local variables Exceptions and error handling Context managers and with blocks 6 

ABOUT CONCURRENCY EXPOSITION 7 

WHAT IS CONCURRENCY? Threads! Promises! Mutexes! Deadlocks! Multiple independent ows of execution Communication between ows 8 

spawn: goto FOR CONCURRENCY (Don’t worry too much about the syntax. Erlang is weird.) %% This is a recursive function with two clauses. %% It counts backwards because that's less code. say(_, 0) -> ok; say(Msg, N) -> io:fwrite("~w ~s~n", [N, Msg]), timer:sleep(1000), say(Msg, N-1). main(_) -> spawn(fun() -> say("task1", 5) end), spawn(fun() -> say("task2", 5) end), timer:sleep(6000). 1 2 3 4 5 6 7 8 9 10 11 12 13 say(_, 0) -> ok; say(Msg, N) -> io:fwrite("~w ~s~n", [N, Msg]), timer:sleep(1000), say(Msg, N-1). %% This is a recursive function with two clauses. 1 %% It counts backwards because that's less code. 2 3 4 5 6 7 8 9 main(_) -> 10 spawn(fun() -> say("task1", 5) end), 11 spawn(fun() -> say("task2", 5) end), 12 timer:sleep(6000). 13 main(_) -> spawn(fun() -> say("task1", 5) end), spawn(fun() -> say("task2", 5) end), timer:sleep(6000). %% This is a recursive function with two clauses. 1 %% It counts backwards because that's less code. 2 say(_, 0) -> 3 ok; 4 say(Msg, N) -> 5 io:fwrite("~w ~s~n", [N, Msg]), 6 timer:sleep(1000), 7 say(Msg, N-1). 8 9 10 11 12 13 %% This is a recursive function with two clauses. %% It counts backwards because that's less code. say(_, 0) -> ok; say(Msg, N) -> io:fwrite("~w ~s~n", [N, Msg]), timer:sleep(1000), say(Msg, N-1). main(_) -> spawn(fun() -> say("task1", 5) end), spawn(fun() -> say("task2", 5) end), timer:sleep(6000). 1 2 3 4 5 6 7 8 9 10 11 12 13 9 

ASYNCIO CALLS IT create_task import asyncio async def print_lines(msg, n): for i in range(n): print(i+1, msg) await asyncio.sleep(0.2) async def main(): await print_lines("basically sync", 3) t1 = asyncio.create_task(print_lines("task1", 3)) t2 = asyncio.create_task(print_lines("task2", 6)) await t1 await t2 asyncio.run(main()) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 async def print_lines(msg, n): for i in range(n): print(i+1, msg) await asyncio.sleep(0.2) import asyncio 1 2 3 4 5 6 7 async def main(): 8 await print_lines("basically sync", 3) 9 t1 = asyncio.create_task(print_lines("task1", 3)) 10 t2 = asyncio.create_task(print_lines("task2", 6)) 11 await t1 12 await t2 13 14 asyncio.run(main()) 15 async def main(): await print_lines("basically sync", 3) t1 = asyncio.create_task(print_lines("task1", 3)) t2 = asyncio.create_task(print_lines("task2", 6)) await t1 await t2 import asyncio 1 2 async def print_lines(msg, n): 3 for i in range(n): 4 print(i+1, msg) 5 await asyncio.sleep(0.2) 6 7 8 9 10 11 12 13 14 asyncio.run(main()) 15 import asyncio async def print_lines(msg, n): for i in range(n): print(i+1, msg) await asyncio.sleep(0.2) async def main(): await print_lines("basically sync", 3) t1 = asyncio.create_task(print_lines("task1", 3)) t2 = asyncio.create_task(print_lines("task2", 6)) await t1 await t2 asyncio.run(main()) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 10 

WHERE DO ERRORS GO? import asyncio async def raise_error(msg): await asyncio.sleep(0.1) raise Exception(msg) async def main(): t1 = asyncio.create_task(raise_error("task1")) # Never awaited. t2 = asyncio.create_task(raise_error("task2")) # Awaited later. await asyncio.create_task(raise_error("task3")) # Awaited now. await t2 asyncio.run(main()) 1 2 3 4 5 6 7 8 9 10 11 12 13 t1 = asyncio.create_task(raise_error("task1")) # Never awaited. import asyncio 1 2 async def raise_error(msg): 3 await asyncio.sleep(0.1) 4 raise Exception(msg) 5 6 async def main(): 7 8 t2 = asyncio.create_task(raise_error("task2")) # Awaited later. 9 await asyncio.create_task(raise_error("task3")) # Awaited now. 10 await t2 11 12 asyncio.run(main()) 13 t2 = asyncio.create_task(raise_error("task2")) # Awaited later. await t2 import asyncio 1 2 async def raise_error(msg): 3 await asyncio.sleep(0.1) 4 raise Exception(msg) 5 6 async def main(): 7 t1 = asyncio.create_task(raise_error("task1")) # Never awaited. 8 9 await asyncio.create_task(raise_error("task3")) # Awaited now. 10 11 12 asyncio.run(main()) 13 await asyncio.create_task(raise_error("task3")) # Awaited now. import asyncio 1 2 async def raise_error(msg): 3 await asyncio.sleep(0.1) 4 raise Exception(msg) 5 6 async def main(): 7 t1 = asyncio.create_task(raise_error("task1")) # Never awaited. 8 t2 = asyncio.create_task(raise_error("task2")) # Awaited later. 9 10 await t2 11 12 asyncio.run(main()) 13 import asyncio async def raise_error(msg): await asyncio.sleep(0.1) raise Exception(msg) async def main(): t1 = asyncio.create_task(raise_error("task1")) # Never awaited. t2 = asyncio.create_task(raise_error("task2")) # Awaited later. await asyncio.create_task(raise_error("task3")) # Awaited now. await t2 asyncio.run(main()) 1 2 3 4 5 6 7 8 9 10 11 12 13 11 

STRUCTURED CONCURRENCY Tasks spawned in a block must nish in that block Execution ow that humans can reason about Nested contexts and task-local state Well-de ned cancellation and error handling No orphaned tasks or missing results 12 

USING TRIO DEVELOPMENT 13 

READ THE DOCS https://trio.readthedocs.io/ No, seriously, the docs are excellent Okay, I’ll talk about trio a bit more 14 

FROM THE VERY BEGINNING Async code can call sync code Sync code can’t call async code import trio def square(n): return n * n async def cube(n): return n * square(n) trio.run(cube, 2) 1 2 3 4 5 6 7 8 9 … but don’t call anything that blocks … but frameworks like Trio are a special case 15 

GETTING IT WRONG Debugging a forgotten await is annoying. Trio provides some debugging tools: trio.abc.Instrument import trio async def slow_square(n): await trio.sleep(0.2) return n * n def foo(): print(await slow_square(3)) # SyntaxError async def bar(): print(slow_square(3)) # RuntimeWarning logged 1 2 3 4 5 6 7 8 9 10 11 print(await slow_square(3)) # SyntaxError import trio 1 2 async def slow_square(n): 3 await trio.sleep(0.2) 4 return n * n 5 6 def foo(): 7 8 9 async def bar(): 10 print(slow_square(3)) # RuntimeWarning logged 11 print(slow_square(3)) # RuntimeWarning logged import trio 1 2 async def slow_square(n): 3 await trio.sleep(0.2) 4 return n * n 5 6 def foo(): 7 print(await slow_square(3)) # SyntaxError 8 9 async def bar(): 10 11 16 

NURSERIES: ACTUAL CONCURRENCY A nursery block doesn’t exit until all child tasks are complete. import trio async def print_lines(msg, n): for i in range(n): print(i+1, msg) await trio.sleep(0.2) print("task finished:", msg) async def main(): print("parent: started!") async with trio.open_nursery() as nursery: nursery.start_soon(print_lines, "task1", 4) nursery.start_soon(print_lines, "task2", 2) print("parent: waiting for tasks to finish...") print("parent: all done!") trio.run(main) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 async def print_lines(msg, n): for i in range(n): print(i+1, msg) await trio.sleep(0.2) print("task finished:", msg) import trio 1 2 3 4 5 6 7 8 async def main(): 9 print("parent: started!") 10 async with trio.open_nursery() as nursery: 11 nursery.start_soon(print_lines, "task1", 4) 12 nursery.start_soon(print_lines, "task2", 2) 13 print("parent: waiting for tasks to finish...") 14 print("parent: all done!") 15 16 trio.run(main) 17 async with trio.open_nursery() as nursery: nursery.start_soon(print_lines, "task1", 4) nursery.start_soon(print_lines, "task2", 2) print("parent: waiting for tasks to finish...") import trio 1 2 async def print_lines(msg, n): 3 for i in range(n): 4 print(i+1, msg) 5 await trio.sleep(0.2) 6 print("task finished:", msg) 7 8 async def main(): 9 print("parent: started!") 10 11 12 13 14 print("parent: all done!") 15 16 trio.run(main) 17 17 

TIMEOUTS import trio async def main(): with trio.move_on_after(3): print("Went to sleep") await trio.sleep(2) print("Still sleeping") await trio.sleep(2) print("Woke up on my own") print("Nap time is over") try: with trio.fail_after(2): async with open_nursery() as n: n.start_soon(trio.sleep, 1) n.start_soon(trio.sleep, 3) await trio.sleep(4) except trio.TooSlowError as e: print(f"Took too long: {e!r}") trio.run(main) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 with trio.move_on_after(3): print("Went to sleep") await trio.sleep(2) print("Still sleeping") await trio.sleep(2) print("Woke up on my own") print("Nap time is over") import trio 1 2 async def main(): 3 4 5 6 7 8 9 10 11 try: 12 with trio.fail_after(2): 13 async with open_nursery() as n: 14 n.start_soon(trio.sleep, 1) 15 n.start_soon(trio.sleep, 3) 16 await trio.sleep(4) 17 except trio.TooSlowError as e: 18 print(f"Took too long: {e!r}") 19 20 trio.run(main) 21 try: with trio.fail_after(2): async with open_nursery() as n: n.start_soon(trio.sleep, 1) n.start_soon(trio.sleep, 3) await trio.sleep(4) except trio.TooSlowError as e: print(f"Took too long: {e!r}") import trio 1 2 async def main(): 3 with trio.move_on_after(3): 4 print("Went to sleep") 5 await trio.sleep(2) 6 print("Still sleeping") 7 await trio.sleep(2) 8 print("Woke up on my own") 9 print("Nap time is over") 10 11 12 13 14 15 16 17 18 19 20 trio.run(main) 21 import trio async def main(): with trio.move_on_after(3): print("Went to sleep") await trio.sleep(2) print("Still sleeping") await trio.sleep(2) print("Woke up on my own") print("Nap time is over") try: with trio.fail_after(2): async with open_nursery() as n: n.start_soon(trio.sleep, 1) n.start_soon(trio.sleep, 3) await trio.sleep(4) except trio.TooSlowError as e: print(f"Took too long: {e!r}") trio.run(main) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 18 

ERRORS AND CANCELLATION import trio async def print_lines(msg, n): for i in range(n): print(i+1, msg) await trio.sleep(0.4) async def timebomb(delay): await trio.sleep(delay) raise Exception("KABOOOM!") async def main(): try: async with trio.open_nursery() as n: n.start_soon(print_lines, "child", 5) n.start_soon(timebomb, 1) await print_lines("body", 5) except Exception as e: print(f"Error: {e}") trio.run(main) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 19 

OTHER FEATURES Task-local storage Communication between tasks Events Channels Locks, semaphores, … Async generators (with caveats) Threads (if you must) Async lesystem operations Subprocesses 20 

CONCLUSION CODA 21 

WHAT HAVE WE LEARNED? goto is bad (but we already knew this) spawn is bad, no matter how you spell it Structured Concurrency is great Use Trio for your next async project 22 

APPLAUSE!!! THE END 23 

QUESTIONS? 24