From the docs:

The SequenceMatcher class has this constructor:

class difflib.SequenceMatcher(isjunk=None, a='', b='', autojunk=True)

The problem in your code is that by doing

seq=difflib.SequenceMatcher(a,b)

you are passing a as value for isjunk and b as value for a, leaving the default '' value for b. This results in a ratio of 0.0.

One way to overcome this (already mentioned by Lennart) is to explicitly pass None as extra first parameter so all the keyword arguments get assigned the correct values.

However I just found, and wanted to mention another solution, that doesn't touch the isjunk argument but uses the set_seqs() method to specify the different sequences.

>>> import difflib
>>> a = 'abcd'
>>> b = 'ab123'
>>> seq = difflib.SequenceMatcher()
>>> seq.set_seqs(a.lower(), b.lower())
>>> d = seq.ratio()*100
>>> print d
44.44444444444444