class InsertionSort:
    def __init__(self):
        self._data = None
        
    def sort(self, data):
        """
        Perform an insertion sort of a list of data.
        """
        self._data = data
        
        for p in range(1, len(self._data)):
            j = p
            
            while (j > 0) and (self._data[j] < self._data[j-1]):
                self._data[j-1], self._data[j] = \
                    self._data[j], self._data[j-1]  # swap
                j -= 1
                
        self._verify_sorted()

    def _verify_sorted(self):
        """
        Verify that the data list 
        has been successfully sorted.
        """
        for k in range(1, len(self._data)):
            value = self._data[k]
            prev  = self._data[k - 1]
            
            if value < prev:
                raise Exception('Sort error: '
                                f'{value} < {prev} at {k}')