Dynamically registering reactive handlers at runtime

[Merlijn Sebrechts]

Hi Alex


Thank you for your response, we did not realize this was the case, this
might come in handy!

Now to get some clarification: If we have the following code:

if some-condition:
    @when(...)
    def some_function(...):
        ...

and `some-condition` changes after the initial registration of handlers.
Will `some_function` be registered, or will this only happen during the
next hook run?

The possible values for package-names are not known during build time. We
do know that _all_ queued packages have to be installed. Would it be
possible to have an `apt.installed.all` state?



Kind regards
Merlijn Sebrechts

2016-02-18 10:39 GMT+01:00 Alex Kavanagh <alex.kavanagh at canonical.com>:

>
>
> On Thu, Feb 18, 2016 at 7:23 AM, Stuart Bishop <
> stuart.bishop at canonical.com> wrote:
>
>> On 12 February 2016 at 22:55, Merlijn Sebrechts
>> <merlijn.sebrechts at gmail.com> wrote:
>> > Hi all
>> >
>> >
>> > We have a layer that can install a number of different packages. It
>> decides
>> > at runtime which packages will be installed. This layer uses the `apt`
>> layer
>> > which sets a state `apt.installed.<packagename>`.
>> >
>> > We want to react to this installed state, but we don't know what the
>> > packagename will be beforehand. This gets decided at runtime.
>> Therefore, we
>> > can't put it in a @when() decorator.
>> >
>> > Is there a way to dynamically register handlers at runtime?
>>
>
> From a Python perspective, there's nothing stopping you, at the top level
> in the file, or in a function, doing:
>
> if some-condition:
>     @when(...)
>     def some_function(...):
>         ...
>
> i.e. decorators don't have to be at the top level in a module/file - they
> are essentially 'just' syntactic sugar for a function call on the defined
> function.
>
> @when(...)
> def something():
>     ....
>
> is conceptually equivalent to:
>
> def something():
>     ...
>
> when(...)(something)
>
> However, I'd caution against dynamically defining the handlers, as it
> might make the program/script less easy to reason about, test and debug.
>
> I'm guessing that the possible values for packagename are known at build
> time, so could you not just either list them all with their appropriate
> handlers, or bundle them all into a single @when(...) if they share a
> common handler?
>
> e.g.
>
> @when('apt.installed.packagename1')
> def ...
>
> @when('apt.installed.packagename2')
> def ...
>
> and/or
>
> @when('apt.installed.packagename1', 'apt.installed.packagename2', ...)
> def ...
>
> Explicit tends to be more helpful than implicit.  Hope this is useful
> and/or helpful!
> Alex.
>
> --
> Alex Kavanagh - Software Engineer
> Cloud Dev Ops - Solutions & Product Engineering - Canonical Ltd
>
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