# Async Process

This is considered an advanced topic.

## Synchronous versus Asynchronous

Most program code operates synchronously. This means that each statement in your code gets processed and finishes before the next can begin. This makes for easy-to-understand code. It is also a requirement in many cases - a subsequent piece of code often depend on something calculated or defined in a previous statement.

Consider this piece of code in a traditional Python program:

        print("before call ...")
        long_running_function()
        print("after call ...")

When run, this will print `"before call ..."`, after which the `long_running_function` gets to work for however long time. Only once that is done, the system prints `"after call ..."`. Easy and logical to follow. Most of Evennia work in this way and often it’s important that commands get executed in the same strict order they were coded.

Evennia, via Twisted, is a single-process multi-user server. In simple terms this means that it swiftly switches between dealing with player input so quickly that each player feels like they do things at the same time. This is a clever illusion however: If one user, say, runs a command containing that `long_running_function`, all other players are effectively forced to wait until it finishes.

Now, it should be said that on a modern computer system this is rarely an issue. Very few commands run so long that other users notice it. And as mentioned, most of the time you want to enforce all commands to occur in strict sequence.

When delays do become noticeable and you don’t care in which order the command actually completes, you can run it asynchronously. This makes use of the `run_async()` function in `src/utils/utils.py`:

        run_async(function, *args, **kwargs)

Where `function` will be called asynchronously with `*args` and `**kwargs`. Example:

        from evennia import utils
        print("before call ...")
        utils.run_async(long_running_function)
        print("after call ...")

Now, when running this you will find that the program will not wait around for `long_running_function` to finish. In fact you will see `"before call ..."` and `"after call ..."` printed out right away. The long-running function will run in the background and you (and other users) can go on as normal.

## Customizing asynchronous operation

A complication with using asynchronous calls is what to do with the result from that call. What if `long_running_function` returns a value that you need? It makes no real sense to put any lines of code after the call to try to deal with the result from `long_running_function` above - as we saw the `"after call ..."` got printed long before `long_running_function` was finished, making that line quite pointless for processing any data from the function. Instead one has to use callbacks.

`utils.run_async` takes reserved kwargs that won’t be passed into the long-running function:

- `at_return(r)` (the callback) is called when the asynchronous function (`long_running_function` above) finishes successfully. The argument `r` will then be the return value of that function (or `None`).

          def at_return(r):
              print(r)

- `at_return_kwargs` - an optional dictionary that will be fed as keyword arguments to the `at_return` callback.

- `at_err(e)` (the errback) is called if the asynchronous function fails and raises an exception. This exception is passed to the errback wrapped in a Failure object `e`. If you do not supply an errback of your own, Evennia will automatically add one that silently writes errors to the evennia log. An example of an errback is found below:

            def at_err(e):
                print("There was an error:", str(e))

- `at_err_kwargs` - an optional dictionary that will be fed as keyword arguments to the `at_err` errback.

An example of making an asynchronous call from inside a Command definition:

        from evennia import utils, Command

        class CmdAsync(Command):

           key = "asynccommand"

           def func(self):

               def long_running_function():
                   #[... lots of time-consuming code  ...]
                   return final_value

               def at_return_function(r):
                   self.caller.msg(f"The final value is {r}")

               def at_err_function(e):
                   self.caller.msg(f"There was an error: {e}")

               # do the async call, setting all callbacks
               utils.run_async(long_running_function, at_return=at_return_function,
    at_err=at_err_function)

That’s it - from here on we can forget about `long_running_function` and go on with what else need to be done. Whenever it finishes, the `at_return_function` function will be called and the final value will pop up for us to see. If not we will see an error message.

## delay

The `delay` function is a much simpler sibling to `run_async`. It is in fact just a way to delay the execution of a command until a future time. This is equivalent to something like `time.sleep()` except delay is asynchronous while `sleep` would lock the entire server for the duration of the sleep.

         from evennia.utils import delay

         # [...]
         # e.g. inside a Command, where `self.caller` is available
         def callback(obj):
            obj.msg("Returning!")
         delay(10, callback, self.caller)

This will delay the execution of the callback for 10 seconds. This function is explored much more in the Command Duration Tutorial.

You can also try the following snippet just see how it works:

    @py from evennia.utils import delay; delay(10, lambda who: who.msg("Test!"), self)

Wait 10 seconds and ‘Test!’ should be echoed back to you.

## The @interactive decorator

As of Evennia 0.9, the `@interactive` [decorator](https://realpython.com/primer-on-python- decorators/) is available. This makes any function or method possible to ‘pause’ and/or await player input in an interactive way.

        from evennia.utils import interactive

        @interactive
        def myfunc(caller):

          while True:
              caller.msg("Getting ready to wait ...")
              yield(5)
              caller.msg("Now 5 seconds have passed.")

              response = yield("Do you want to wait another 5 secs?")

              if response.lower() not in ("yes", "y"):
                  break

The `@interactive` decorator gives the function the ability to pause. The use of `yield(seconds)` will do just that - it will asynchronously pause for the number of seconds given before continuing. This is technically equivalent to using `call_async` with a callback that continues after 5 secs. But the code with `@interactive` is a little easier to follow.

Within the `@interactive` function, the `response = yield("question")` question allows you to ask the user for input. You can then process the input, just like you would if you used the Python `input` function. There is one caveat to this functionality though - it will only work if the function/method has an argument named exactly `caller`. This is because internally Evennia will look for the `caller` argument and treat that as the source of input.

All of this makes the `@interactive` decorator very useful. But it comes with a few caveats. Notably, decorating a function/method with `@interactive` turns it into a Python generator. The most common issue is that you cannot use `return <value>` from a generator (just an empty `return` works). To return a value from a function/method you have decorated with `@interactive`, you must instead use a special Twisted function `twisted.internet.defer.returnValue`. Evennia also makes this function conveniently available from `evennia.utils`:

        from evennia.utils import interactive, returnValue

        @interactive
        def myfunc():

            # ...
            result = 10

            # this must be used instead of `return result`
            returnValue(result)

## Assorted notes

Overall, be careful with choosing when to use asynchronous calls. It is mainly useful for large administration operations that have no direct influence on the game world (imports and backup operations come to mind). Since there is no telling exactly when an asynchronous call actually ends, using them for in-game commands is to potentially invite confusion and inconsistencies (and very hard-to-reproduce bugs).

The very first synchronous example above is not really correct in the case of Twisted, which is inherently an asynchronous server. Notably you might find that you will not see the first `before call ...` text being printed out right away. Instead all texts could end up being delayed until after the long-running process finishes. So all commands will retain their relative order as expected, but they may appear with delays or in groups.

## Further reading

Technically, `run_async` is just a very thin and simplified wrapper around a Twisted Deferred object; the wrapper sets up a default errback also if none is supplied. If you know what you are doing there is nothing stopping you from bypassing the utility function, building a more sophisticated callback chain after your own liking.