Comparing two strings for partial or full overlapping characters

Question

I'm trying to compare two sets of string for partial or complete overlapping substrings and then finding how much the overlap is between them. This is what I have so far but I have a few unit tests that aren't passing. Any help would be appreciated.

# The function findLargestOverlap receives two parameters target and candidate, both strings.
# If either strand is empty or the strands are not the same length, return -1.
# It should find the largest overlap and return the size of the overlap.

def findLargestOverlap(target, candidate):
    if (len(target) == 0 or len(candidate) == 0) or (len(target) != len(candidate)):
        return -1
    elif len(target) == len(candidate):
        return 0
    else:
        match = SequenceMatcher(None, target, candidate).find_longest_match(0, len(target), 0, len(candidate))
        return len(match)

Below are my Unittests

"""
Do Not Edit this file. You may and are encouraged to look at it for reference.
"""

import unittest
import dnaSequencing

class TestFindLargestOverlap(unittest.TestCase):
    def test001_findLargestOverlapExists(self):
        self.assertTrue('findLargestOverlap' in dir(dnaSequencing),
                    'Function "findLargestOverlap" was not defined, check your spelling')

def test002_targetStrandEmpty(self):
    from dnaSequencing import findLargestOverlap
    self.assertEqual(findLargestOverlap('', 'bbb'), -1,
                     'The target strand is blank so the function should return a value of -1')

def test003_candidateStrandEmpty(self):
    from dnaSequencing import findLargestOverlap
    self.assertEqual(findLargestOverlap('aaa', ''), -1,
                     'The candidate strand is blank so the function should return a value of -1')

def test004_bothStrandsEmpty(self):
    from dnaSequencing import findLargestOverlap
    self.assertEqual(findLargestOverlap('', ''), -1,
                     'Both strands are blank so the function should return a value of -1')

def test005_strandsAreNotEqualLengths1(self):
    from dnaSequencing import findLargestOverlap
    self.assertEqual(findLargestOverlap('aa', 'bbb'), -1,
                     'The target and candidate strings are different lengths so the function should return a value of -1')

def test006_strandsAreNotEqualLengths2(self):
    from dnaSequencing import findLargestOverlap
    self.assertEqual(findLargestOverlap('aaa', 'bb'), -1,
                     'The target and candidate strings are different lengths so the function should return a value of -1')

def test007_zeroOverlap(self):
    from dnaSequencing import findLargestOverlap
    self.assertEqual(findLargestOverlap('aaa', 'bbb'), 0, 'There is no overlap between the strings "aaa" and "bbb"')

def test008_zeroOverlap(self):
    from dnaSequencing import findLargestOverlap
    self.assertEqual(findLargestOverlap('abccccba', 'bbccccbb'), 0, 'There is no overlap between the strings "abccccba" and "bbccccbb"')

def test009_partialOverlap(self):
    from dnaSequencing import findLargestOverlap
    self.assertEqual(findLargestOverlap('abcdefg', 'defgabc'), 4, 'There is an overlap of "defg" (4 characters)')

def test010_completeOverlap(self):
    from dnaSequencing import findLargestOverlap
    self.assertEqual(findLargestOverlap('aaaaaaaa', 'aaaaaaaa'), 8, 'There is an overlap of "aaaaaaaa" (8 characters)')

if __name__ == '__main__':
    unittest.main()