How to use mock_open() with patch.object() in test annotation

Question

I'm trying to mock read from file. Using examples it can be done with a construction like:

with patch('__builtin__.open', mock_open(read_data='1'), create=True) as m:
    with open('foo') as h:
        result = h.read()

I wonder, is there a way to mock open function using my testcase annotation. Like:

@patch.object(__builtin__, 'open')
def test_check_status_running(self, m_open):

I don't find the correct way, because for me it works for int and doesn't work for strings:

@patch.object(__builtin__, 'open')
def test_check_status_running1(self, m_open):
    m_open = mock_open(read_data='1')
    pid = open('testfile').read().strip()
    print type(pid)                    # <class 'mock.MagicMock'>
    self.assertEqual(1, int(pid))      # Pass
    self.assertEqual('1', pid)         # Fails MismatchError: '1' != <MagicMock name='open().read()' id='39774928'>

See http://stackoverflow.com/a/3268310/552793 for example of mocking file open using the @patch annotation — jlb83, Dec 11 '14 at 17:59

score 21 · Accepted Answer · answered Nov 29 '16 at 15:59

You can patch the open method in many ways. I prefer to patch the builtins.open and to pass the mocked object to the test method like this:

from unittest.mock import patch, mock_open
from mymodule import method_that_read_with_open

class TestPatch(unittest.TestCase):
    @patch('builtins.open', new_callable=mock_open, read_data='1')
    def test_open_file(self, m):
        string_read = method_that_read_with_open()
        self.assertEqual(string_read, '1')
        m.assert_called_with('filename', 'r')

Note that we are passing the mock_open function without calling it!

But because you are patching the builtin method, you can also do:

class TestPatch(unittest.TestCase):
    @patch('builtins.open', mock_open(read_data='1'))
    def test_open_file(self):
        string_read = method_that_read_with_open()
        self.assertEqual(string_read, '1')
        open.assert_called_with('filename', 'r')

This two examples are basically equivalent: in the first one we are giving to the patch method a factory function that he will invoke to create the mock object, in the second one we are using an already created object as argument.

Finally an example that works with Python 3.5. The mock library has changed and there's a lot of answers out there which only work for older versions :/ — Dirk, Jan 04 '17 at 17:18

score 1 · Answer 2 · answered Sep 15 '16 at 18:45

1

Assign a mock_open instance to the new_callable parameter:

class TestClass(unittest.TestCase):
    @mock.patch('file_tested.open', new_callable=mock.mock_open())
    def test_function(self, mock_open):
        pass

answered Sep 15 '16 at 18:45

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How to use mock_open() with patch.object() in test annotation

2 Answers2