longest alpa str

from collections import namedtuple

def longest_alpha_str(source):

    def chunks(source):
        newstr = prev = ""
        for ch in source + chr(0):
            if ch >= prev:
                newstr += ch
                prev = ch
            else:
                yield newstr
                newstr = prev = ch

    Chunk = namedtuple('Chunk', 'chunk length')
    lengths = (Chunk(chunk, len(chunk)) for chunk in chunks(source))
    return max(lengths, key=lambda chunk: chunk.length).chunk

tests = 'azcbobobegghakl', 'abcdabghikmnastuvwx','abcdefhiabcdefhiklnprst'

for test in tests:
    print(test, '->', longest_alpha_str(test))