The class:

class ABC(object):
  def __init__(self, files):
    self.store = []
    self.parse_files(files)

  def parse_files(files):
    for filename in files:
      with open(filename, newline="") as f:
        self.store.append(f.read())

The test:

from unittest.mock import Mock, mock_open, patch

class TestABC:
  result1 = 'string1'
  result2 = 'string2'
  result = Mock()
  result.side_effect = [result1, result2]

  @patch("builtins.open", new_callable=mock_open, read_data=result())
  def test_parse_files(self, a):
    item = ABC(['foo', 'bar'])
    assert item.store == ['string1', 'string2']

But item.store == ['string1', 'string1']

I thought the open would return result1 and result2 since result() is iter?