elide: cut text at or before max_len, keeping complete words if possible

elide.py

#!/usr/bin/env python3

def elide(text, max_len, ellipsis='…'):
    if len(text) <= max_len:
        return text

    cut = max_len

    while cut > 0 and text[cut - 1].isalnum():
        cut -= 1

    if cut > 0:  # found a place to cut
        if cut == max_len:
            cut -= 1  # discard trailing non-alnum
        return text[:cut].strip() + ellipsis

    else:  # no good place to cut: cut anyway
        return text[:max_len - 1] + ellipsis


def main():
    import sys

    def usage():
        print(f'Usage:  {sys.argv[0]} one or more words 15')
        print('Output: one or more…')
        sys.exit()

    if len(sys.argv) < 3:
        usage()

    try:
        max_len = int(sys.argv[-1])
    except ValueError:
        usage()

    text = ' '.join(sys.argv[1:-1])

    print(elide(text, max_len))


if __name__ == '__main__':
    main()

test_elide.py

from elide import elide

def test_shorter():
    text = 'Simple text'
    assert text == elide(text, 12)

def test_exact():
    text = 'Simple text'
    assert text == elide(text, 11)

def test_elide():
    text = 'Simple text'
    want = 'Simple…'
    assert want == elide(text, 10)

def test_elide_hyphenated():
    text = 'Error-prone'
    want = 'Error-…'
    assert want == elide(text, 10)

def test_elide_after_hyphen():
    text = 'Error-prone'
    want = 'Error-…'
    assert want == elide(text, 7)

def test_elide_at_hyphen():
    text = 'Error-prone'
    want = 'Error…'
    assert want == elide(text, 6)

def test_elide_number():
    text = 'War of 1914'
    want = 'War of…'
    assert want == elide(text, 10)

def test_elide_midword():
    text = 'Incomprehensible'
    want = 'Incompreh…'
    assert want == elide(text, 10)