[Tutor] Re: slots - a more basic issue

[Lloyd Kvam]

We've been concerned about attribute name errors for our Record classes
which tie back to database tables.  Our solution was to have a class
attribute named fieldlist with the list of fields from the associated
database table.  Then we used __getattr__, __setattr__, and __delattr__
to enforce the requirement that all attributes had to be listed in the
class's fieldlist.  This avoids doing special handling elsewhere in the
class methods.

Here is a simplified example where the fieldlist is wired into the class
definition.  This approach may work for you.

class Example(object):
     fieldlist = ('a','b','c')

     def __getattr__(self, key):
         if key in self.fieldlist:    # provide default value of None
             return None
         else:
             raise AttributeError("%s is not an attribute of this object" % key)

     def __setattr__(self, key, value):
         if key in self.fieldlist:
             self.__dict__[key] = value
         else:
             raise AttributeError("%s is not an attribute of this object" % key)

     def __delattr__(self, key):
         raise AttributeError( "Attribute %s must not be deleted" % key)

I gave this some light testing and it seemed to work.  HTH.



Arthur Siegel wrote:

> Dman writes -
> 
> 
>>Use a method signature such as :
>>            def __init__(self, x, y, color=None ):
>>and you'll get :
>>
> 
>>Traceback (most recent call last):
>> File "<stdin>", line 1, in ?
>>TypeError: __init__() got an unexpected keyword argument 'kolor'
>>
> 
> Yeah, but I have lots of potential attributes, and lots
> of inheritance relationships - which makes the method signature
> unwieldy - just what **kw seems to be made for.
> 
> Looks like I'll end up with building a list of attributes and something
> like:
> 
> for key in kws:
>    if key not in optlist:
>       print 'WARNING  " %s" not a valid keyword for %s'
> %(key,self.__class__.__name__)
> 
> Just found a bunch of typos in my demos doing this approach.
> 
> Art
> 
> 
> 
> _______________________________________________
> Tutor maillist  -  Tutor@python.org
> http://mail.python.org/mailman/listinfo/tutor
> 
> 


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Lloyd Kvam
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