--- Marcus Goldfish <[EMAIL PROTECTED]> wrote: > Are there guidelines for when properties should be used vs. instance > variables? For example, it often seems more convenient to directly > use instance variables instead of properties, like MyClass2 listed > below. Is one class more pythonic than the other?
Hello Marcus One guideline for using properties is to simply ask yourself: do I need to run other code when getting, setting or deleting this attribute? Here's a demo of what I mean, based on your first example class: >>> class MyClass1(object): ... def __init__(self, value=0): ... self.__value = value ... def getValue(self): ... # log attribute usage ... print repr(self),': value accessed' ... return self.__value ... def setValue(self, value): ... # validate new values ... if not isinstance(value, int) or value < 0: ... raise ValueError, 'positive integer required' ... self.__value = value ... def delValue(self): ... # reset to 0, instead of deleting ... self.__value = 0 ... value = property(getValue, setValue, delValue) ... >>> c = MyClass1() >>> c.value = -1 Traceback (most recent call last): File "<stdin>", line 1, in ? File "<stdin>", line 11, in setValue ValueError: positive integer required >>> c.value = 9 >>> print c.value <__main__.MyClass1 object at 0x402b6c6c> :value accessed 9 >>> del c.value >>> v = c.value <__main__.MyClass1 object at 0x402b6c6c> : value accessed >>> print v 0 >>> Note that you could also do all of this by calling the methods directly. For example, you could set the value to 1 by calling self.setValue(1) or delete it by calling self.delValue(). The nice thing about using properties, though, is that everything is accessed through a single attribute name. So if there are lots of attributes which need special get/set/del handling, you'll end up with much cleaner code. As to which way of doing things is the most pythonic: well, as always, it depends on what you are trying to achieve - if your code is not doing what you want it to do, then that is about as unpythonic as it can get :-) HTH John Ridley Send instant messages to your online friends http://uk.messenger.yahoo.com _______________________________________________ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
