Simple questions on use of objects (probably faq)

[bruno at modulix]

Brian Elmegaard wrote:
> Hi,
> 
> I am struggling to understand how to really appreciate object
> orientation. I guess these are FAQ's but I have not been able to find
> the answers. Maybe my problem is that my style and understanding are
> influenced by matlab and fortran.
> 
> I tried with the simple example below and ran into several questions:
> 1: Why can't I do:
>     def __init__(self,self.x):
>   and avoid the self.x=x

Before you assign a value to it, self.x does not exists.

> 2: Is it a good idea to insert instances in a list or is there a simpler
> way to do something with all instances of a given type?

Depends on your program. But there are at least ways to make this a bit
more transparent, see below.

> 3: Why canøt I say and get the maximum of instance attributes and a
> list of them?  
> y_max=max(y[].x) and 

y_max = max([s.x for s in y])

> ys=[y[].x]

ys = [s.x for s in y]

> 4: Can I avoid the dummy counter i in the for loop and do something
> like: 
> yz=[y[:-1].x-y[1:].x]

based on the other snippet you posted:
> yz=[]
> for i in range(len(ys)-1):
>     yz.append(ys[i+1]-ys[i])

yz = [next - current for next, current in zip(ys[1:], ys[0:-1])]

I'm not sure this is exactly what you want, but this should get you
started anyway.


Now how you could do it the OO way (Q&D, not really tested):

class Foo(object):
  _instances = []

  def __init__(self, x):
    self.x = x # no, you won't avoid it
    self._add_instance(self)

  @classmethod
  def _add_instance(cls, instance):
     cls._instances.append(instance)

  @classmethod
  def get_instances(cls):
    return cls._instances[:]

  @classmethod
  def get_values(cls):
    return [i.x for i in cls.get_instances()]

  @classmethod
  def max(cls):
    return max(cls.get_values())

  @classmethod
  def min(cls):
    return min(cls.get_values())

  @classmethod
  def strange_computation(cls):
    values = cls.get_values()
    return [next - current \
             for next, current in zip(values[1:], values[:-1])]


for value in [10.0, 110.0, 60.0]:
  Foo(value)

Foo.get_values()
Foo.max()
Foo.min()
Foo.strange_computation()


HTH
-- 
bruno desthuilliers
python -c "print '@'.join(['.'.join([w[::-1] for w in p.split('.')]) for
p in 'onurb at xiludom.gro'.split('@')])"