Re: [Twisted-Python] automat question

[Chris Withers]

I'm not sure we're quite on the same page: I'm not looking to inspect 
the state, but more be notified when certain edges are traversed.


That said: what's the best practice when you want to visualize the 
current state of a machine? In this case, the machine is for a unit of 
work in a scheduling app, and the UI has a list of these units where 
it'd make sense to show the current state.


The unit of work also forms the boundary API between the from end object 
model and the pool/scheduling back end. As such, each side can naturally 
call inputs methods on the machine, as I believe automat intends, but 
it's how to get notified by an output?


So, the front end may want to say "I no longer need this UoW done, 
please kill it", the back end then needs to run the code to kill 
whatever's running the UoW.
Likewise, when the backend finishes a UoW, it wants to call either 
machine.result(...) or machine.error(...) to say what happened, but how 
do I wire in the front end bit that needs to get called when this happens?


cheers,

Chris

On 07/03/2021 22:34, Glyph wrote:
Automat is designed to make this sort of thing intentionally annoying, 
as you have discovered:).


The idea is that if you want to know this sort of internal state, it’s 
for a specific reason. That's not a blanket "No" — see for example how 
automat deals with serialization — but each such interface should be 
minimal and thoughtfully designed. Otherwise a state machine library 
just becomes a bunch of complex infrastructure around making an 
arbitrary series of function calls, and loses all of its helpful 
formalisms.


So probably we do need to make a change to automat if you really need 
to do this, but first it's important to know what your use-case is. In 
a lot of cases the answer is "just make an output. Stop trying to make 
the application see into the guts of the framework." But without 
knowing it’s impossible to say!


-g

Hi,

Apologies if there's a better list for this, please let me know where...

I've grown to like Glyph's automat package a lot, but in a current
project, I have observers that need to know when a machine changes state.

What's the best way to let those observers know?

cheers,

Chris

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[Twisted-Python] Strange recursion error with twisted.web

[Peter Westlake]

I'm getting a "maximum recursion depth exceeded" error that appears to be 
coming from flatten(). The odd thing is that it only happens sometimes. The 
HTML that's being flattened does have a few Deferreds in it. Those come from 
function calls, which cache the results, which might explain why I only see the 
error on the first visit to the page (as far as I can tell). 

The system recursion limit is the standard 1000. My HTML is only nested a few 
tags deep, two orders of magnitude short of that. Is there anything about the 
way flatten() works that might cause this behaviour?

Peter.

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Re: [Twisted-Python] automat question

[Glyph]

> On Mar 9, 2021, at 3:18 AM, Chris Withers  wrote:
> 
> I'm not sure we're quite on the same page: I'm not looking to inspect the 
> state, but more be notified when certain edges are traversed.

The way you get notified when an edge is traversed is you add an output to that 
edge :).

When I was talking about "internal state", what I meant was not the literal 
current state atom of the machine, but the presence and names of states and 
edges themselves.  Within the abstraction of the state machine, the way you 
find out that an edge was traversed by having an output run.

> That said: what's the best practice when you want to visualize the current 
> state of a machine? In this case, the machine is for a unit of work in a 
> scheduling app, and the UI has a list of these units where it'd make sense to 
> show the current state.

This is definitely a gap.  I think today the best way to do this would be to 
have an event emitted by an output that invokes the serialization code to 
discover the state and have the UI show that.

> The unit of work also forms the boundary API between the from end object 
> model and the pool/scheduling back end. As such, each side can naturally call 
> inputs methods on the machine, as I believe automat intends, but it's how to 
> get notified by an output?

The output can just... call a UI observer?

> So, the front end may want to say "I no longer need this UoW done, please 
> kill it", the back end then needs to run the code to kill whatever's running 
> the UoW.
> Likewise, when the backend finishes a UoW, it wants to call either 
> machine.result(...) or machine.error(...) to say what happened, but how do I 
> wire in the front end bit that needs to get called when this happens?

Again... have outputs.  I think there's something you're leaving out about how 
you want to have some generalized output, but without knowing a bit more it's 
hard to say how it could help more :).

-g

> 
> cheers,
> 
> Chris
> 
> On 07/03/2021 22:34, Glyph wrote:
>> Automat is designed to make this sort of thing intentionally annoying, as 
>> you have discovered:).
>> 
>> The idea is that if you want to know this sort of internal state, it’s for a 
>> specific reason. That's not a blanket "No" — see for example how automat 
>> deals with serialization — but each such interface should be minimal and 
>> thoughtfully designed. Otherwise a state machine library just becomes a 
>> bunch of complex infrastructure around making an arbitrary series of 
>> function calls, and loses all of its helpful formalisms.
>> 
>> So probably we do need to make a change to automat if you really need to do 
>> this, but first it's important to know what your use-case is. In a lot of 
>> cases the answer is "just make an output. Stop trying to make the 
>> application see into the guts of the framework." But without knowing it’s 
>> impossible to say!
>> 
>> -g
>>> Hi,
>>> 
>>> Apologies if there's a better list for this, please let me know where...
>>> 
>>> I've grown to like Glyph's automat package a lot, but in a current 
>>> project, I have observers that need to know when a machine changes state.
>>> 
>>> What's the best way to let those observers know?
>>> 
>>> cheers,
>>> 
>>> Chris
>>> 
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Re: [Twisted-Python] Strange recursion error with twisted.web

[Glyph]

> On Mar 9, 2021, at 4:54 AM, Peter Westlake  wrote:
> 
> I'm getting a "maximum recursion depth exceeded" error that appears to be 
> coming from flatten(). The odd thing is that it only happens sometimes. The 
> HTML that's being flattened does have a few Deferreds in it. Those come from 
> function calls, which cache the results, which might explain why I only see 
> the error on the first visit to the page (as far as I can tell). 
> 
> The system recursion limit is the standard 1000. My HTML is only nested a few 
> tags deep, two orders of magnitude short of that. Is there anything about the 
> way flatten() works that might cause this behaviour?

flatten() can definitely result in some deep recursive stacks, particularly in 
combination with synchronous Deferreds which have their own accumulating stack 
costs. I'd be interested to see a minimal reproducer for this though, I'm sure 
we could do a lot better.

-g


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Re: [Twisted-Python] Strange recursion error with twisted.web

[Peter Westlake]

On Tue, 9 Mar 2021, at 19:28, Glyph wrote:
> 
> 
> > On Mar 9, 2021, at 4:54 AM, Peter Westlake  wrote:
> > 
> > I'm getting a "maximum recursion depth exceeded" error that appears to be 
> > coming from flatten(). The odd thing is that it only happens sometimes.
> 
> flatten() can definitely result in some deep recursive stacks, 
> particularly in combination with synchronous Deferreds which have their 
> own accumulating stack costs. I'd be interested to see a minimal 
> reproducer for this though, I'm sure we could do a lot better.

It’s good to know that this isn’t completely unexpected. I’ll try to reproduce 
it, thanks.

Peter.

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