show differences in strings -- uses SequenceMatcher

stringdiff.py

from difflib import SequenceMatcher

def diff(actual, expected):
  if actual != expected:
    raise AssertionError(combine(actual, expected))

def combine(a,b):
  """Form one string showing the differences between a and b."""
  sm = SequenceMatcher(None, a,b)
  out = []
  for op in sm.get_opcodes():
    if op[0] == 'equal':
      out.append(sm.a[op[1]:op[2]])
    else:
      out.append("("+sm.a[op[1]:op[2]]+"|"+sm.b[op[3]:op[4]]+")")
  return ''.join(out)

aa="Beware the angry badger with a cudgel"
ab="Beware any badger with a stout cudgel"

# sample output: 
# Beware (the |)an(gr|)y badger with a (|stout )cudgel

if __name__ == '__main__':
  print combine(aa,ab)