Python 3, an illustrated tour Transcripts
Chapter: Asynchronous Programming
Lecture: Walk-through: Asyncio Generators

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0:00 In this video, we're going to look at async gen test, open that up in your editor.
0:04 And this is about making asynchronous generators.
0:07 So it says write an a synchronous generator countdown
0:10 that accepts a count and a delay, when looped over asynchronously,
0:13 it returns the numbers from countdown to an including zero
0:16 it waits for delay seconds before returning the next value.
0:19 This should be very familiar to you if you've already done async iter test.
0:24 So let's make a generator that does this
0:27 the point of this is to show that generators are typically easier to implement
0:31 and easier to debug than iterators.
0:34 So in order to make an a synchronous generator
0:37 we say async def and it's going to be called countdown,
0:40 it's going to accept count and delay
0:43 and I'm just going to go into a while loop here,
0:46 I am going to say while 1 and I will yield count
0:51 and if count equals 0 then I will break out of here.
0:58 Otherwise, I will say count minus equals 1 and then I'm going to sleep
1:04 and I could just say time.sleep, but that's not going to be an asynchronous sleep.
1:08 So in order to do an asynchronous sleep,
1:10 I need to import the asyncio library and then we need to await it.
1:16 So we're going to call await asyncio.sleep.
1:22 and we're going to sleep for delay seconds
1:25 again because I am in an asynchronous generator,
1:29 I can call await on an asynchronous function here
1:33 and that should be it, let's give it a test and make sure that it works here.
1:37 So just run it here, it takes a while because it's doing some delaying here,
1:44 but it looked like it ran.
1:46 This is the same test code basically as async iter test
1:49 and note that it takes 2 seconds to run
1:51 and if we come down here we say that we're counting down from 2 with 1 second delay.
1:55 So that should take 2 seconds to run.
1:57 You'll note that we're sort of unrolling the asynchronous iteration protocol down here
2:02 and asserting that we raise a stop async iteration error
2:06 You'll note that we don't explicitly raise that exception
2:09 but when the generator returns that raises the exception for us,
2:12 so thanks for watching this video.
2:14 Hopefully you have a better feel for those a synchronous generators now
2:18 and you can hopefully take advantage of those and use those in your code.