adding properties dynamically (how to?)

Bruno Desthuilliers bdesth.quelquechose at free.quelquepart.fr
Sun Aug 17 13:09:20 CEST 2008


André a écrit :
> I didn't want to hijack the original thread but I have basically the
> same request...
> 
> On Aug 17, 7:09 am, Bruno Desthuilliers
> <bdesth.quelquech... at free.quelquepart.fr> wrote:
>> akonsu a écrit :> hello,
>>
> [SNIP]
> 
>> Wrong solution to your problem, I'd say. Let's start again:
>>
>> """
>>  > i need to add properties to instances dynamically during run time.
>>  > this is because their names are determined by the database contents.
>> """
>>
>> Care to elaborate ? I may be wrong, but I suspect you're trying to roll
>> your own python/database mapper. If so, there are quite a couple Python
>> ORMs around. Else, please tell us more.
> 
> I'm not the original poster, but I'd like to do the same thing (for a
> different reason).
> 
> I have a program (crunchy) that is extensible via plugins.  New
> options available via plugins can be turned on or off (or selected
> among a list of options).  I have a module for user preferences (let's
> call it prefs.py) that allows the setting of these options (and do
> error checking, automatic saving of the options selected for future
> sessions, etc.).  These options are implemented as properties.
> 
> Currently I have it simplified so that only two lines need to be added
> to prefs.py to add new options; something like
> options = { ...
>     'new_option': [value1, value2, ..., valueN],
> ...}
> 
> and
> class Preferences(object):
> ...
> 
>    new_option = make_property('new_option', 'some nicely worded help
> string')
> 
> ===
> make_property is a custom define function that return fgets, fsets,
> fdel and doc.

You may want to write your own descriptor object instead. The 'property' 
  class is just one possible way to use the descriptor protocol for 
"smart" attributes.

> Ideally, I'd like to be able to define new would-be properties from
> the plugin and add them to the class prior to creating instances.  In
> other words, have something like
> 
> ===
> for option in options_defined_in_plugins:
>    add_option_as_property_to_Preferences(Preferences, option, ...)
> 
> user_preferences = Preferences()

Not the same problem as the OP here. Your properties are not 
per-instance, so just adding them to the class is ok. Remember that in 
Python, classes are objects too, and (a couple corner cases set aside) 
nothing prevent you from dynamically adding attributes to an object. The 
following snippets are equivalent:

class Foo1(object):
     attrib = object()

class Foo2(object):
     pass

Foo2.attrib = object()

class Foo3(object):
     pass

setattr(Foo3, 'attrib', object())





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