Re: API for setting/getting values on Dexterity content

Perhaps you did not read my blog posts or you did not got my point.

The developer experience with Plone and Dexerity - if done the
right way - is horrible. As developer right now I have to know
which behavior implements which field in order to set/get
a value the right way. It is horrible because as a developer
I want to write pythonic code (translates to attribute access).
This was also one of the design goals and promises of Dexterity
as far as I recall. But reality is different. Everyone one write
Pythonic code but the code is potentially broken (see my rants
about the plone.app.event implementation).

So the idea - or better the necessity - is to give developers
a consistent way to read/set values. This means in the first place that
the whole behavior stuff should be hidden from the developer - at least
for the standard developer which average skills who is the potential
user of plone.api. As a developer (e.g. in my current role as writer
of a fat migration process) I want assign values to an object without
checking other peoples source code about where a particular attribute
is defined and check for the related interface. This complexity must be
hidden in order to writer code in more clear way, in a more reliable way
and in a more readable way.

The implementation of set/get methods must perform some kind of
introspection on the schema and the attached behaviors in order
to retrieve the related interface for adopting the current context
object. This is of course in some way slower that direct attribute
access but it is the right way. There is certainly a performance loss
with this approach but I guess that this could be minimized with
clever caching of the schema introspection results.

-aj

Roel Bruggink wrote:
> Hi Andreas,
> 
> I get what you say. I have a few questions on how this would work in
> practice, though.
> 
> Assuming the following:
> IMySchema():
> start
> end
> name
> animal_type default='Känguru'
> 
> IMyFirstBehavior():
>  start
>  end
>  lives_in default='Känguru Island'
> 
>  IMySecondBehavior():
>  name default='Jos Henken'
> 
> How should set_value('start', now()) and get_value('start') behave?
> 
> Would a dict-like api be of more use, ie obj['IMyBehavior.fieldname'] =
> 'new value' ? We'll still probably have to adapt the object to that api,
> though.
> 
> So in the end, direct attribute access for just the schema itself would
> work, unless the fieldname is unique between all behaviors.
> 
> -Roel
> 
> On 22 October 2014 10:25, Andreas Jung
> <[email protected]
> <mailto:[email protected]>> wrote:
> 
>     Hi there,
> 
>     the following suggestion is based on former blog posts of mine, e.g.
> 
>     https://www.andreas-jung.com/contents/bad-dexterity-application-design
>     https://www.andreas-jung.com/contents/copying-dexterity-fields-from-one-instance-to-another-instance
> 
>     I clearly see a need for an API for safely setting and getting values
>     for Dexerity content that takes all aspects of behaviors into account.
>     In general: direct attribute access to on Dexterity instance was a nice
>     goal but in reality it is a broken and dangerous programming pattern in
>     Plone right now - unless the direct getting/setting of values through
>     direct attribute access can be fixed somehow (I think this is very
>     hard).
> 
>     The safest way would be to provide methods like
> 
>     plone.api.content.set_value(obj, fieldname,  value)
>     value = plone.api.content.get_value(obj, fieldname)
> 
>     as part of plone.api or through methods of the Dexterity base
>     classes Item and Container:
> 
>     obj.set_value(fieldname, value)
>     value = obj.get_value(fieldname).
> 
>     The method names are dust and smoke and could be named differently.
> 
>     Any thoughts on the need for encapsulating the setting/getting of value?
>     Any thoughts where such an API would fit best (plone.api vs. base
>     classes)?
> 
>     -aj
> 
> 
> 
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> 
> 
> -- 
> Roel Bruggink
> http://www.fourdigits.nl/mensen/roel-bruggink
> 
> Four Digits BV
> http://www.fourdigits.nl <http://www.fourdigits.nl/> tel: +31(0)26 4422700
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