[Discussion] Re. Substring counting

[Gribouillis]

Here is a version

import unittest as ut
import re

def subsequence_count(needle, haystack):
    if len(needle) == 1:
        return haystack.count(needle)
    elif len(needle) > 1:
        a, b = needle[0], needle[1:]
        result = 0
        for mo in re.finditer(a, haystack):
            result += subsequence_count(b, haystack[mo.end():])
        return result
    else:
        raise ValueError(('Non empty string expected, got', needle))




class TestSubsequence(ut.TestCase):
    def test_banana(self):
        n = subsequence_count('an', 'banana')
        self.assertEqual(n, 3)
        
    def test_panama(self):
        n = subsequence_count('an', 'trans-Panamanian banana')
        self.assertEqual(n, 25)

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