[Tutor] when to use properties?

[John Ridley]

--- Marcus Goldfish <magoldfish at gmail.com> 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

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