Peter Cacioppi <peter.cacio...@gmail.com> writes: > I was laboring under some misconception that there was Python magic > that allowed __init__ and only __init__ to add class attributes by > setting their values. Good to know this piece of magic isn't part of > Python, and thus lazy eval can be handled more cleanly than I > originally thought.
Rather, the magic is that ‘__init__’ will be called automatically by the class creation protocol, to initialise any new instance of that class. So the method to write if you want instance-specific initialisation is still ‘__init__’. > Think about it this way. None here really means "not yet initialized". > It is a value that cannot occur naturally and thus functions as a > not-initialized flag. As you point out, though, that's not a menaing of None agreed on everywhere. You can't expect that the caller will not be passing a value of None for that parameter. > So this, somewhat arbitrary, context sensitive value should be isolated > as much as possible. You don't want it popping up hither and yon, you > want to type as infrequently as possible and localized it to as few > methods as possible. Right. The pattern you're looking for is called a “sentinel value” <URL:https://en.wikipedia.org/wiki/Sentinel_value>. Usually, the None singleton is good enough for a sentinel value, but often it has a meaning already in your context, so is not available as a sentinel value. In cases where None is not available, make a specific value that has no other meaning, and use that specifically for your parameter's sentinel value:: class Foo(object): _default_for_x = object() def __init__(self, x=_default_for_object): """ Initialise a new instance. """ if x is self._default_for_object: # No value for ‘x’ was passed, because ‘_default_for_x’ # would not be passed by accident. x = self._get_value_for_x() self.x = x def _get_default_value_for_x(self): """" Calculate the default ‘x’ value for this instance. """ return some_computation(self) -- \ “When a well-packaged web of lies has been sold to the masses | `\ over generations, the truth will seem utterly preposterous and | _o__) its speaker a raving lunatic.” —Dresden James | Ben Finney -- https://mail.python.org/mailman/listinfo/python-list
