Newbie again: computing attributes on the fly

Clarence Gardner clarence at
Wed Jun 16 17:19:05 CEST 1999

Olaf Delgado (delgado at Mathematik.Uni-Bielefeld.DE) wrote:
: Hi everybody,
: here's another couple of questions to the enlightened ones. I'd like to
: compute certain properties of a class instance only once and store it for
: future use. I'd like to do those computations on demand only, because some
: might be really, really expensive. My first try looks like this:
: class x:
:     def __getattr__(self, attr):
:         import string
:         if string.find(attr, "get_") != 0:
:             val = ( lambda x = getattr(self, "get_"+attr)(): x )
:             self.__dict__[attr] = val
:             return val
:         else:
:             raise AttributeError("no such attribute")
:     def get_lost(self):
: 	return 0 # really, really expensive :)
:     def get_a_life(self):
: 	return 1

On occasion I've done things like this (not necessarily for cost reasons,
but I just might want to do something different the first time).  What
I've done is:

class x:
	def dosomething(self, arg):
		initial blah, blah, blah
		self.dosomething = self._dosomething2
	def _dosomething2(self, arg):
		second_and_subsequent blah, blah, blah
Clarence Gardner
AvTel Communications
Software Products and Services Division
clarence at

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