mock.mock_open works as expected when reading the entire file (read()) or when reading a single line (readline()), but it seems to not support reading a number of bytes (read(n)).These work as expected:    from mock import mock_open, patch    # works: consume entire "file"    with patch('__main__.open', mock_open(read_data='bibble')) as m:        with open('foo') as h:            result = h.read()    assert result == 'bibble'  # ok    # works: consume one line    with patch('__main__.open', mock_open(read_data='bibble\nbobble')) as m:        with open('foo') as h:            result = h.readline()    assert result == 'bibble\n'  # okBut trying to read only a few bytes fails--mock_open returns the entire read_data instead:    # consume first 3 bytes of the "file"    with patch('__main__.open', mock_open(read_data='bibble')) as m:        with open('foo') as h:            result = h.read(3)    assert result == 'bib', 'result of read: {}'.format(result)  # failsOutput:    Traceback (most recent call last):      File "/tmp/t.py", line 25, in <module>        assert result == 'bib', 'result of read: {}'.format(result)    AssertionError: result of read: bibbleThe unfortunate effect of this is that mock_open cannot be used with pickle.load.    with open('/path/to/file.pkl', 'rb') as f:        x = pickle.load(f)  # this requires f.read(1) to work

Confirmed that the behavior exists in Python 3.6 as well.

Internally mock_open implementation uses line based iteration [0] to keep track of the state change between read calls. So readline too ignores the argument. There isissue25690 for an alternate mock_open implementation but the code change is large and adds a lot of features. A simpler approach would be to use StringIO or BytesIO to keep track of state changes and they provide read, readline and readlines API. mock_open docs mentions about using a customized mock for complex cases but I don't know if worthy enough to make this enhancement given the internal implementation change to support the API or to document this behavior. Looking further there also seems to be a test case for it [1] which will fail if this is fixed since this returns all characters instead of first 10 like using open().read(10).    def test_mock_open_read_with_argument(self):        # At one point calling read with an argument was broken        # for mocks returned by mock_open        some_data = 'foo\nbar\nbaz'        mock = mock_open(read_data=some_data)        self.assertEqual(mock().read(10), some_data)$ echo -n 'foo\nbar\nbaz' > /tmp/a.txt$ ./python.exePython 3.8.0a0 (heads/master:f5107dfd42, Dec 16 2018, 13:41:57)[Clang 7.0.2 (clang-700.1.81)] on darwinType "help", "copyright", "credits" or "license" for more information.>>> with open('/tmp/a.txt') as f:...     actual = f.read(10)...     actual, len(actual)...('foo\nbar\nba', 10)>>> with open('/tmp/a.txt') as f:...     from unittest.mock import mock_open...     mock = mock_open(read_data=f.read())...     mock_data = mock().read(10)...     mock_data, len(mock_data)...('foo\nbar\nbaz', 11)[0]https://github.com/python/cpython/blob/f5107dfd42121ef40b13eb678705802f0ff02cf9/Lib/unittest/mock.py#L2349[1]https://github.com/python/cpython/blob/f5107dfd42121ef40b13eb678705802f0ff02cf9/Lib/unittest/test/testmock/testwith.py#L284

The off-by-one error in a test added for an unrelated issue (#17467) makes me think @michael.foord just made a mistake in the test.> mock_open docs mentions about using a customized mock for complex casesI think it's more for complex things like fetching data using a callback for example, this seems like a normal use case for mock_open(). Even more so, leaving the behavior as it is now may lead someone to think there is a bug in its code.I opened a new PR to fix this issue:https://github.com/python/cpython/pull/11521

New changeset11a8832c98b3db78727312154dd1d3ba76d639ec by Chris Withers (Rémi Lapeyre) in branch 'master':bpo-31855: unittest.mock.mock_open() results now respects the argument of read([size]) (GH-11521)https://github.com/python/cpython/commit/11a8832c98b3db78727312154dd1d3ba76d639ec

New changeseta6516f89aa0f416c7514ac364bb48ac7d1455487 by Chris Withers (Miss Islington (bot)) in branch '3.7':bpo-31855: unittest.mock.mock_open() results now respects the argument of read([size]) (GH-11521) (#13152)https://github.com/python/cpython/commit/a6516f89aa0f416c7514ac364bb48ac7d1455487

PR was merged and backported to 3.7. So I am closing this as fixed. Thanks Rémi for the patch.