'should return blocks only blocks that pass ``block_filter_fn``'

def test_block_filter_fn(self):

    def is_not_indented(line):
        strip_diff = (len(line) - len(line.lstrip()))
        return (strip_diff == 0)
    def no_tw(block):
        if block[0].startswith('Tw'):
            return None
        return block
    (contents, provider, data) = self.contents_provider_and_data(strip_lines=False, strip_newlines=True, new_block_delim_fn=is_not_indented, block_filter_fn=no_tw)
    self.assertEqual(data, [['One', '                ABCD'], ['Three']])
    self.assertCounters(provider, 3, 2, 2)
