How to use a class property to store function variables?

alex23 wuwei23 at gmail.com
Wed Apr 28 04:20:35 CEST 2010


GZ <zyzhu2... at gmail.com> wrote:
> I do not think it will help me. I am not trying to define a function
> fn() in the class, but rather I want to make it a "function reference"
> so that I can initialize it any way I like later.

It always helps to try an idea out before dismissing it out of hand.
Experimentation in the interpreter is cheap and easy.

>>> class A(object):
...   fn = staticmethod(lambda x: x*x)
...
>>> A.fn(10)
100
>>> A.fn = staticmethod(lambda x: x**x)
>>> A.fn(3)
27
>>> def third(x): return x/3
...
>>> A.fn = staticmethod(third)
>>> A.fn(9)
3

However, I'm assuming you're wanting to do something like this:

>>> class B(object):
...   def act(self):
...     print self.fn()

That is, providing a hook in .act() that you can redefine on demand.
If so, note that you only need to decorate functions as staticmethods
if you're assigning them to the class. If you intend on overriding on
_instances_, you don't:

>>> B.fn = staticmethod(lambda: 'one') # assign on class
>>> b = B() # instantiate
>>> b.act() # act on instance
one
>>> B.fn = staticmethod(lambda: 'two') # assign on class
>>> b.act() # existing instance calls new version on class
two
>>> b.fn = staticmethod(lambda: 'three') # assign on instance
>>> b.act()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in act
TypeError: 'staticmethod' object is not callable
>>> b.fn = lambda: 'three' # look Ma, no staticmethod!
>>> b.act()
three

Incidentally, this is known as the Strategy pattern, and you can see a
simple example of it in Python here: http://en.wikipedia.org/wiki/Strategy_pattern#Python

Hope this helps.



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