Q4.3.6Sorting a list containing None
The built-in function sorted and sequence method sort require that the elements in the sequence be of types that can be compared: they will fail, for example, if a list contains a mixture of strings and numbers. However, it is frequently the case that a list contains numbers and the special value, None (perhaps denoting missing data). Devise a way to sort such a list by passing a lambda function in the argument key; the None values should end up at the end of the sorted list.
Show solution
One solution is to construct a tuple with two elements for each item in the list to be sorted: first, a boolean value indicating whether the item is None or not; second, the value itself. When these tuples are sorted, the first element is False for all the numbers (which can be compared as the second element) and True for all the None values. Since False always compares as "less than" True, there is no need to compare different types:
>>> lst = [4, 2, 6, None, 0, 8, None, 3]
>>> lst.sort(key=lambda e: (e is None, e))
>>> lst
[0, 2, 3, 4, 6, 8, None, None]