Mock call to write()

Question

I have a logger class that needs to write strings to file. So, I have a method like so:

def write_to_file(self, string):
  self.__file_handle.write(string)

Note that error handling has been edited out. Clearly I want to test this without writing to a file. Thus mocks via Mock. I have seen this which explains how to mock open but it does not help me here -- I open the file_handle in __init__. Now, I can do that in setUp() but the mock_open seems to go out of scope after setUp and thus is of no use in the test case.

How would you write a test method to test the write_to_file method using Mock?

score 8 · Accepted Answer · edited Oct 29 '18 at 21:00

8

I reached here from google, only to realize after a while that this has been builtin in mock since 1.0 using mock_open

edited Oct 29 '18 at 21:00

Noam Hacker

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answered Sep 12 '13 at 09:50

lonetwin

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score 4 · Answer 2 · edited Jul 03 '13 at 22:15

Simple really...

from mock import patch

def setUp(self):
    [...]
    mock_file_handle = Mock()
    with patch('__builtin__.open') as mock_file_handle:
        self.sut = Logger()
    [...]

def test_write(self):
    [...]
    self.sut.write_message_to_file("ook?")
    self.assertTrue(self.sut.file_handle.write.called)
    self.assertTrue(self.sut.file_handle.flush.called)
    [...]

If anyone has a better solution, please let me know...

score 1 · Answer 3 · answered Nov 11 '11 at 14:35

1

I would start with the ability to change __file_handle in the __init__. As if you replace this with any mock / fake object that has the .write signature you can test what is written to it.

Maybe by creating a method that sets up the __file_handle and then overwriting the method for this test?

answered Nov 11 '11 at 14:35

Andrew Cox

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That is one way. But it should be possible to mock .write() as well. – Sardathrion - against SE abuse Nov 11 '11 at 15:17

Mock call to write()

3 Answers3