Re: [Rest2web-develop] sections and configspec

[Michael F. <mi...@pc...>]

Nicola Larosa wrote:
>>>The only thing really missing now is having "lists of sections", for the
>>>cases when their number cannot be determined in advance. Maybe some easy
>>>workaround can be found.
> 
> 
>>I don't understand what you want. If you give me an example I'll think
>>about it. It sounds like you need a way of referencing (naming)
>>individual sections. Is this just for the programmer - or for the system
>>administrator (the user).
> 
> 
> My cousin has several cats and several dogs, and wants to "configure" them.
> (I know, peculiar cousin.) I don't exactly know how many they are, and
> moreover, their number could change with time.
> 
> All the cats have the same params among them, the same with dogs, but cats'
> params are slightly different from dogs' params.
> 
> I write this configspec:
> 
> [ cats ]
>     [[ cat ]]
>         color = string
>         whiskers = int
> [ dogs ]
>     [[ dog ]]
>         color = string
>         trained = boolean
> 
> Then I write this config file:
> 
> [ cats ]
>     [[ felix ]]
>         color = black
>         whiskers = 5
>     [[ miao ]]
>         color = white
>         whiskers = 7
> [ dogs ]
>     [[ fido ]]
>         color = brown
>         trained = yes
>     [[ lassie ]]
>         color = grey
>         trained = no
>     [[ rintintin ]]
>         color = pink
>         trained = no
> 
> How do I specify that the "cat" configspec has to be used for the "felix"
> and "miao" sections, while the "dog" configspec has to be used for the
> "fido", "lassie" and "rintintin" sections?
> 
> Or better, how do I specify that, when reading the config file, all
> subsections of the "cats" section have to be validated against the "cat"
> configspec, and that all subsections of the "dogs" section have to be
> validated against the "dog" configspec?
> 

You want named types of configspec sections and to be able to apply
those types in your config file.

In this particular example you are saying that all 'dogs' (sub-sections
in the dogs section) should use the [[dog]] configspec and 'cats'
(sub-sections in the cats section) should use the 'cat' configspec.

Two possible implementations spring to mind - named sections, or
repeated sections. Repeated sections would meet your specific example
here - but aren't a generic solution. I'll suggest an alternative
implementation of named sections, that is less complicated than yesterdays.

Named Sections
==============

The trouble is that with a hierarchical structure (rather than a flat
one) there is no unique identifier per section (sections have non unique
names).

The alternatives are either to allow sections to be named - a magic
value (either '__name__' or '__type__') or to use something like a
dotted syntax to reference sections 'section.sub-section.sub-sub-section'.

I think my previous explanation was overly complex. I favour this
solution - using a '__type__' magic value in sections. These can be
treated specially when parsing configspecs and are not treated specially
by the ``write`` method.

I write this configspec:

[ cats ]
    [[ cat ]]
	__type__ = cat
        color = string
        whiskers = int
[ dogs ]
    [[ dog ]]
	__type__ = dog
        color = string
        trained = boolean

Then I write this config file:

[ cats ]
    [[ felix ]]
	__type__ = cat
        color = black
        whiskers = 5
    [[ miao ]]
	__type__ = cat
        color = white
        whiskers = 7
[ dogs ]
    [[ fido ]]
	__type__ = dog
        color = brown
        trained = yes
    [[ lassie ]]
	__type__ = dog
        color = grey
        trained = no
    [[ rintintin ]]
	__type__ = dog
        color = pink
        trained = no

When parsing a configspec any __type__ values are not treated as checks,
but a *separate* dictionary of types is kept.

If the validate method encounters a section with a __type__ value it
looks for the configspec for this section in the type dictionary.

There are a couple of implementation questions if you like this
solution.....

Repeated sections could solve this particular example without the use of
 a magic value in the config file.

You could do something like

[ cats ]
    [[ __repeated__ ]]
        color = string
        whiskers = int
[ dogs ]
    [[ __repeated__ ]]
	__type__ = dog
        color = string
        trained = boolean

Effectively saying that any subsection of 'cats' or 'dogs' has to meet
the __repeated__ specification. This isn't useful if you want to be able
to have a 'dog' somewhere else of course.....

Using the dotted syntax allows you to avoid magic values in your
configspec (which is less useful).

You could do :

[ dogs ]
    [[ fido ]]
	__type__ = dogs.dog
        color = brown
        trained = yes
    [[ lassie ]]
	__type__ = dogs.dog
        color = grey
        trained = no
    [[ rintintin ]]
	__type__ = dogs.dog
        color = pink
        trained = no

I don't like this personally.....

Regards,

Fuzzy
http://www.voidspace.org.uk/python