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']?

Answer 1

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