def sum_items(a_list):
    """
    @purpose Returns the sum of all items in a_list, and return 0 if empty list
    @param
        a_list: a list of items passed to this function to sum
    @complexity:
        Worst Case - O(N): When summing all values of a list with N items
        Best Case - O(1): when passing empty list or encounter error
    @precondition Passing a list with numerical values
    @postcondition Return the sum of those numerical values in list, or 0 if it's empty
    """
    try:
        if len(a_list) > 0:
            sum = 0
            for i in range(len(a_list)):
                sum += a_list[i]
            return sum
        else:
            return 0
    except TypeError:
        return "Please only insert numerical type lists."