Distributing items across columns

coldist.py

from math import ceil

class ColumnDistributor(list):
    """Distributes a list of items across a given number of columns.

    >>> items = list('ABCDEFGHIJKLM')
    >>> columns = ColumnDistributor(items)

    >>> columns.distribute(4)
    >>> for c in columns:
    ...     print c
    ['A', 'B', 'C', 'D']
    ['E', 'F', 'G', 'H']
    ['I', 'J', 'K', 'L']
    ['M']

    >>> columns.distribute(3)
    >>> for c in columns:
    ...     print c
    ['A', 'B', 'C', 'D', 'E']
    ['F', 'G', 'H', 'I', 'J']
    ['K', 'L', 'M']

    If there's less items than the number of columns, the remaining columns
    get represented as empty lists:

    >>> columns = ColumnDistributor(list('AB'))
    >>> columns.distribute(3)
    >>> columns
    [['A'], ['B'], []]
    """

    def __init__(self, items):
        super(ColumnDistributor, self).__init__(items)
        self._items = items

    def distribute(self, num_cols):
        columns = []

        # Max items any column will contain
        items_per_col = int(ceil(float(len(self._items)) / num_cols))

        for n in range(num_cols):
            start = items_per_col * n
            end = items_per_col * (n + 1)
            columns.append(self._items[start:end])

        # Store the resulting columns by calling superclass constructor
        super(ColumnDistributor, self).__init__(columns)


if __name__ == "__main__":
    import doctest
    doctest.testmod()