Sorting by attributes that can be None

sorted(items, key=lambda i: i.data.value if i.data else 0)

Use as key a tuple, like (False, value). If value is None, then the tuple should be (True, None).

Tuples are compared by their first element first, then the second, et cetera. False sorts before True. So all None values will be sorted to the end.

def none_to_end_key(item):
    value = item.data.value if item.data else None
    return (value is None, value)

sorted(items, key=none_to_end_key)

Will sort all None values to the end.

I see now that you have tagged your question Python-2.7, then this is probably overkill. In Python 3, comparing None to an integer or string raises an exception, so you can't simply sort a list with None and other values, and something like this is needed.

just filter for the None before sorting

sorted(filter(None, items), key=attrgetter('data.value'))