Python Class design: attributes

Question

I constructed a class:

class Foo (object):
      def __init__(self,List):
          self.List=List
      @property
      def numbers(self):
          L=[]
          for i in self.List:
              if i.isdigit():
                 L.append(i)
          return L
      @property
      def letters(self):
          L=[]
          for i in self.List:
              if i.isalpha():
                 L.append(i)
          return L

>>> inst=Foo(['12','ae','45','bb'])
>>> inst.letters
['ae', 'bb']
>>> inst.numbers
['12', '45']

How can I add attributes so I could do inst.numbers.odd that would return ['45']?

Pierre GM · Accepted Answer · 2012-09-13 15:23:12Z

Your numbers property returns a list, so a numbers.odd won't work.

However, you could follow a workflow like:

define a small class Numbers, that would define two properties even and odd For example, Numbers could take a list as argument of its __init__, the even property would return only the even number of this list [i for i in List if int(i)%2 == 0] (and odd the odd ones)...
create an instance of Numbers in your Foo.numbers property (using your Foo.List to initialize it) and return this instance...

Your Numbers class could directly subclass the builtin list class, as suggested. You could also define it like

class Numbers(object):
    def __init__(self,L):
        self.L = L
    @property
    def even(self):
        return [i for i in self.L if not int(i)%2]
    def __repr__(self):
        return repr(self.L)

Here, we returning the representation of Numbers as the representation of its L attribute (a list). Fine and dandy until you want to append something to a Numbers instance, for example: you would have to define a Numb.append method... It might be easier to stick with making Numbers a subclass of list:

 class Numbers(list):
    @property
    def even(self):
        ...

Edited: corrected the // by a %, because I went too fast and wasn't careful enough

yes,but how can i get inst.numbers still return ['12', '45'] then. i couldnt figure it out...
Your even test should be not int(i)%2. (not int(57)//2) is (not int(56)//2) should return True.

mgilson · Accepted Answer · 2012-09-13 15:30:00Z

2

Here's a silly example:

class mylst(list):
    @property
    def odd(self):
        return [ i for i in self if int(i)%2 == 1 ]

class Foo(object):
    def __init__(self,lst):
        self.lst = list(lst)

    @property
    def numbers(self):
        return mylst( i for i in self.lst if i.isdigit() )

a = Foo(["1","2","3","ab","cd"])
print(a.numbers)
print(a.numbers.odd)

Basically, we just subclass list and add a property odd which returns another list. Since our structure is a subclass of list, it is virtually indistinguishable from the real thing (Horray duck typing!). mylst.odd could even return a new instance of mylst if you wanted to be able to filter it again (e.g. a.numbers.odd.in_fibinocci )

edited Sep 13, 2012 at 15:30

answered Sep 13, 2012 at 15:17

mgilson

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4 Comments

Jon Clements Over a year ago

I can think of some horrible ways this could be achieved, but inheriting from list is elegant +1

mgilson Over a year ago

@JonClements -- I'm pretty sure you could do something like this using descriptors, but I've never had occasion to use them. And inheriting from python builtin types is easy as pi!

Jon Clements Over a year ago

Yep - descriptors can be used (and if it were more complex, could well be a practical answer) - but in this use-case, it's overkill, and this answer solves the OPs problem nicely ;)

root Over a year ago

@ Jon Clements - you are right, it is a silly example but I was trying to learn smth at more abstract level. I will try to dig into descriptors then...

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