creating many similar properties

Carl Banks pavlovevidence at gmail.com
Wed Oct 18 09:30:19 CEST 2006


Lee Harr wrote:
> I understand how to create a property like this:
>
> class RC(object):
>     def _set_pwm(self, v):
>         self._pwm01 = v % 256
>     def _get_pwm(self):
>         return self._pwm01
>     pwm01 = property(_get_pwm, _set_pwm)
>
>
> But what if I have a whole bunch of these pwm properties?
>
> I made this:
>
> class RC(object):
>     def _makeprop(name):
>         prop = '_%s' % name
>         def _set(self, v):
>             v_new = v % 256
>             setattr(self, prop, v_new)
>         def _get(self):
>             return getattr(self, prop)
>         return property(_get, _set)
>
>     pwm01 = _makeprop('pwm01')
>     pwm02 = _makeprop('pwm02')
>
>
> Not too bad, except for having to repeat the name.
>
> I would like to just have a list of pwm names and
> have them all set up like that. It would be nice if
> each one was set to a default value of 127 also....
>
> Any thoughts?


The metaclass solution.  I use this idiom occasionally, whenever I want
to fiddle with the class dict before letting the type constructor at
it.

class mod256metatype(type):
    def __new__(metatype,name,bases,clsdict):
        for sym in clsdict.get('__mod256__',()):
            prop = '_%s'  % sym
            def _set(self,v):
                setattr(self,prop,v%256)
            def _get(self):
                return getattr(self,prop)
            clsdict[sym] = property(_get,_set)
        return type.__new__(metatype,name,bases,clsdict)

class RC(object):
    __metaclass__ = mod256metatype
    __mod256__ = ["pwm01","pwm02"]


Carl




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