Diff two texts and markup result with <del> and <ins>.

gistfile1.py

 # -*- coding: utf-8 -*-

import re
import difflib
import codecs
from itertools import chain


_diff_split_re = re.compile(r'(\s+)(?u)')


def text_split(text):
    worditer = chain([u''], _diff_split_re.split(text))
    return [x + worditer.next() for x in worditer]


def diff(old_text, new_text):
    old = text_split(old_text)
    new = text_split(new_text)
    matcher = difflib.SequenceMatcher(None, old, new)

    def wrap(tag, words):
        return u'<{0}>{1}</{0}>'.format(tag, u''.join(words))

    for tag, i1, i2, j1, j2 in matcher.get_opcodes():
        if tag == 'replace':
            yield wrap('del', old[i1:i2])
            yield wrap('ins', new[j1:j2])
        elif tag == 'delete':
            yield wrap('del', old[i1:i2])
        elif tag == 'insert':
            yield wrap('ins', new[j1:j2])
        else:
            yield u''.join(old[i1:i2])


def merge_lines(seq):
    if not seq:
        return None

    s = u''
    merged = []

    for line in seq:
        if line != u'':
            s += line + ' '
        else:
            merged.append(s)
            s = u''

    return merged


def render_html_diff(old_text, new_text):
    s = u''
    old = old_text.split('\n')
    new = new_text.split('\n')

    for l1, l2 in zip(merge_lines(old), merge_lines(new)):
        s += u''.join((w for w in diff(l1, l2))) + u'\n\n'

    return s