Splits up a string into pages of a maximum length. Includes pages numbers. Tries not to split in the middle of a word. Good for SMS.

split_up.py

from math import ceil

def split_up(s, length=160):
    """
    Splits the string s into pages of no longer than length. Includes
    pages numbers. Tries not to split in the middle of a word.

    >>> split_up("abc", 5)
    ['abc']
    >>> split_up("abcdef", 5)
    ['a 1/6', 'b 2/6', 'c 3/6', 'd 4/6', 'e 5/6', 'f 6/6']
    >>> split_up("%s %s" % ("a" * 150, "b" * 150), 160) # doctest: +ELLIPSIS
    ['aaa...aaa 1/2', 'bbb...bbb 2/2']
    >>> split_up("a" * 160) == ["a" * 160]
    True
    >>> split_up("a" * 161) # doctest: +ELLIPSIS
    ['aaa...aaa 1/2', 'aaaaa 2/2']
    """
    if len(s) <= length:
        return [s]

    length -= 4  # correct for the length of the page number, len(' 1/3') == 4

    total_pages = int(ceil(len(s)/float(length)))
    assert total_pages < 10, 'only a single digit will work'

    words = s.split()
    words.reverse()

    pages = []
    for i in range(total_pages):
        page = []
        needed = length
        while needed and words:
            word = words.pop()
            if len(word) > needed:
                if page:
                    # Put the word back, it'll go on the next page.
                    words.append(word)
                    break
                else:
                    # There is no choice but to split a word. go ahead
                    # and split the word. Put the extra characters we
                    # couldn't use back into consideration for the next
                    # page.
                    extra = word[needed:]
                    word = word[:needed]
                    words.append(extra)
            page.append(word)
            needed -= len(word)
        page_num = i + 1  # one-based page numbers
        pages.append('%s %s/%s' % (' '.join(page), page_num, total_pages))

    return pages