Python 3, an Illustrated Tour Transcripts
Chapter: Asynchronous Programming
Lecture: asyncio Context Managers
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In this video we're going to talk about asynchronous context managers. These were discussed in pep 492, it came out in Python 3.5.
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Let's look at the protocol for asynchronicity in Python 3. There's a couple protocols that you can implement.
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If you want to define your own class that behaves as a coroutine, you can implement the __await__ method
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that means that you can call await on it and get the results from that. You can also Implement an asynchronous iterator and we'll talk about those,
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and in this section, we're going to talk about asynchronous context managers. So they have a __aenter__ and a __aexit__ method.
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If you're not familiar with context managers, let's briefly discuss what traditional context managers look like,
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again in Python, these are things that you can put in a with statement so I can open a file with a with statement
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and within the indented portion of that block the file is going to be opened and when I exit out of it, it's going to close it for me.
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So the traditional thinking about when to use a context manager is if you have some logic you want to insert before a block
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and some logic that you want to insert after, either one of those makes a good case for using a context manager.
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And in that case you implement a __enter__ and a __exit__ method. Here's a brief example of doing that.
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Here's a silly little context manager called runner. We're going to pass an item into the constructor and then when we enter into the block,
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we're going to assume that the item that we passed in is a dictionary and we're going to set the key running to true,
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when we exit out of that block, we will set the key running to false. Down here below we can see we're actually running this
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and you can see that here inside of this block while we are in this block, the running key on item is set to true
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as soon as we unindent out of that block running is set to false. So right at the start this with corresponds to this __enter__ method
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and when we unindent down here, right before this guy, we are at the __exit__ method, right there.
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So that's how you can control inserting logic before and after. On this page, I'm linking to a nice little project.
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It's a Linux window manager utility, but it allows you to run external processes asynchronously in a context manager.
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Let's look at some of the content of that and we can see that it implements this context manager for asynchronous context managers protocol.
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You can see that there's a __aenter__ method and interestingly enough, because it's defined with async here, we can call await within it.
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So that's sort of the key to the asynchronous context manager is within aenter and aexit, you can call asynchronous co-routines as well.
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And this _AIOPopen guy who again runs a processed asynchronously is awaitable, so we see that he has a __await__ method defined as well.
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And in the constructor here you pass in a coroutine, he's yielding from that and returning from that inside of the aenter there.
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I am not going to dig into much more of what's going on, but I just wanted to make you aware of key difference
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between an asynchronous context manager and a normal context manager. So if you need to do logic from a coroutine inside an enter or exit block,
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you would use an asynchronous context manager because inside of there you can call await. Here's another example, we're just going to look at quickly.
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This is heavily inspired by the async timeout project, I'll link there to the GitHub, but we're defining a class called timeout
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and we're going to use a timeout context manager to be able to timeout co-routines. Here we're just going to show the constructor
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we can pass in a timeout value, how long we want to timeout, we'll pass in a loop. Inside of it, we're going to make it canceled attribute
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that says we have not cancelled what we're running and a handler, this handler will be called if we're going to cancel what's running.
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Here we see the asynchronous protocol being implemented, we see an async def __aenter__, so that's when we enter the block
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and you can see that when we enter the block, we're going to get the current time on the loop and we're going to add the timeout to it.
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And that's when we want to timeout this block. We're going to get a task, we'll show the get task implementation below
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and we'll set a handler, on the loop we'll say call at at some time when we're going to call our self cancel method and that gives us back a handler.
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We also have the __aexit__ and you can see that this is very analogous to the traditional context manager,
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we get back an exception and a value and a traceback if there was an exception thrown from within the context
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and here you can see that if the canceled flag is set, then we raise the asyncio timeout error,
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otherwise, if we have a handler we're going to cancel that handler and set the handler to none and we're going to set the test to none.
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Here we have the implementation of the cancel method. It just calls cancel on the task and sets cancel to true.
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Down below we have the get task function that takes a loop and returns the current task in the block. Here's an example of running that,
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we can see when we use an asynchronous context manager instead of saying just with we have to say async with.
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Here we're saying that we're going to time out after two seconds. We're going to pass in our loop as well,
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and we're just going to sleep for one second here. So this will not timeout, it should print done and after.
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If we change this value in here from 1 to say 3 or some value greater than 2, then this would time out and instead of saying done here,
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it would print timeout and it's going to raise this asyncio timeout error. We can handle that with a try block if we want to
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and do the appropriate thing at that point. So this is a simple example of using a context manager that is asynchronous.
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In this video we talked about asynchronous context managers. Again, the key here is that you implement the protocol
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and the key difference between these context managers and traditional context managers is that you can await from within them.