Turn slice into range

I'm using Python 3.3. I want to get a slice object and use it to make a new range object.

It goes something like that:

>>> class A:
    def __getitem__(self, item):
        if isinstance(item, slice):
            return list(range(item.start, item.stop, item.step))

>>> a = A()
>>> a[1:5:2] # works fine
[1, 3]
>>> a[1:5] # won't work :(
Traceback (most recent call last):
  File "<pyshell#18>", line 1, in <module>
    a[1:5] # won't work :(
  File "<pyshell#9>", line 4, in __getitem__
    return list(range(item.start, item.stop, item.step))
TypeError: 'NoneType' object cannot be interpreted as an integer

Well, the problem is obvious here - range doesn't accept None as a value:

>>> range(1, 5, None)
Traceback (most recent call last):
  File "<pyshell#19>", line 1, in <module>
    range(1, 5, None)
TypeError: 'NoneType' object cannot be interpreted as an integer

But what is not obvious (to me) is the solution. How will I call range so it will work in every case? I'm searching for a nice pythonic way to do it.

Try

def ifnone(a, b):
    return b if a is None else a

class A:
    def __getitem__(self, item):
        if isinstance(item, slice):
            if item.stop is None:
                # do something with itertools.count()
            else:
                return list(range(ifnone(item.start, 0), item.stop, ifnone(item.step, 1)))
        else:
            return item

This will reinterpret .start and .step appropriately if they are None.

Another option could be the .indices() method of a slice. It is called with the number of entries and reinterprets None to the appropriate values and wraps negative values around the given length parameter:

>>> a = slice(None, None, None)
>>> a.indices(1)
(0, 1, 1)
>>> a.indices(10)
(0, 10, 1)
>>> a = slice(None, -5, None)
>>> a.indices(100)
(0, 95, 1)

It depends what you intend to do with negative indices …

Edit 2024:

The easiest way is probably

class A:
    def __init__(self, length):
        self._length = length
    def __len__(self):
        return self._length
    def __getitem__(self, item):
        if isinstance(item, slice):
            return range(*item.indices(self._length))
        else:
            return item

Here you have to provide a length when creating A() which is then used if the slicing needs it.