Setting read_data after calling mock.mock_open() - python-3.x

Question

I am testing a class that in turn uses open. I want to patch open on the test class for simplicity, but set read_data per test. So something like this:
#mock.patch('builtins.open', unittest.mock.mock_open())
class test_TheClassUnderTest:
def test_MakeSureAMethodWorks(self):
open().read_data = "custom data for the test"
I confirmed that tests work when I set read_data in the patch decorator:
#mock.patch('builtins.open', unittest.mock.mock_open(read_data="acid burn and zero cooooooool"))
class test_TheClassUnderTest(unittest.TestCase):
def test_MakeSureAMethodWorks(self):
# ...
self.assertEquals(object.a_method(), expected_value)
I've tried setting the return values of the mock returned from open():
open().read.return_value = "custom data for the test"
open().readline.return_value = "custom data for the test"
but it's not working. Is there a way to set the value of read_data after the mock_open() constructor has been called or, more generally, another way to do this?

Answer 1

You don't need to mock the .return_value for open().read. Use mock_open(read_data="fake data") is enough.
E.g.
main.py:
def main(file_path):
lines = []
with open(file_path) as f:
for line in f:
lines.append(line)
return lines
test_main.py:
import unittest
from unittest.mock import mock_open, patch
from main import main
class TestMain(unittest.TestCase):
def test_main(self):
m = mock_open(read_data="custom data for the test\nsecond line")
with patch('builtins.open', m) as mocked_open:
actual = main('fake/file/path')
self.assertEqual(actual, ['custom data for the test\n', 'second line'])
mocked_open.assert_called_once_with('fake/file/path')
if __name__ == '__main__':
unittest.main()
test result:
⚡ coverage run /Users/dulin/workspace/github.com/mrdulin/python-codelab/src/stackoverflow/65533628/test_main.py && coverage report -m --include='./src/**'
.
----------------------------------------------------------------------
Ran 1 test in 0.010s
OK
Name Stmts Miss Cover Missing
-----------------------------------------------------------------------
src/stackoverflow/65533628/main.py 6 0 100%
src/stackoverflow/65533628/test_main.py 12 0 100%
-----------------------------------------------------------------------
TOTAL 18 0 100%