Question

I am trying to create defaultlist, by analogy with defaultdict. The idea is that sometimes I want a list that is pre-filled (virtually!) with some element at all indices for which the value is not explicitly set. The difference with defaultdict is that I do not actually add the default values to the list, unless I have to extend list due to user assignments.

1) Is there any problem with this design?

2) This is the implementation I’m going to use. Is there any issue with it?

class defaultlist(list):
  def __init__(self, default_factory, arg = ()):
    # note: cannot use arg = None since list(None) is not the same as list()
    # alternatively can use:
    # def __init__(self, default_factory, *args, **kwargs)
    self.default_factory = default_factory
    super().__init__(arg)

  def __getitem__(self, key):
    try:
      return super().__getitem__(key)
    except IndexError:
      return self.default_factory()

  def __setitem__(self, key, value):
    for i in range(len(self), key):
      self.append(self.default_factory())
    self.append(value)

EDIT: Originally, I had two mistakes. Sorry, and thanks for pointing out both errors in the answers.

Answer 1

Problem.

1. Your __init__ function tries to forward all arguments to the base class. However, the way you are writing it can only forward 1 argument, and the constructor will always expect the 2nd argument. The proper way to do that in Python is:

   def __init__(self, default_factory, *args, **kwargs):
       super().__init__(*args, **kwargs)
       self.default_factory = default_factory
   

2. The __setitem__ function tries to fill all not-yet-existing indices, but you have missed the key itself — note that range is exclusive in the end — therefore the super().__setitem__ method will fail. You should use .append in that case.

   def __setitem__(self, key, value):
       try:
           super().__setitem__(key, value)
       except IndexError:
           for i in range(len(self), key):
               self.append(self.default_factory())
           self.append(value)