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' # ok But 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) # fails Output: 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: bibble The 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 is issue25690 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.exe Python 3.8.0a0 (heads/master:f5107dfd42, Dec 16 2018, 13:41:57) [Clang 7.0.2 (clang-700.1.81)] on darwin Type "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 cases I 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 changeset 11a8832c98b3db78727312154dd1d3ba76d639ec 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 changeset a6516f89aa0f416c7514ac364bb48ac7d1455487 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.