bpo-37008: make mock_open handle able to honor next()

[Anvil]

I've reported the issue on https://bugs.python.org/issue37008 and now I'm trying to bring a solution to this minor issue.

I think it could be trivially backported to 3.7 branch.

https://bugs.python.org/issue37008

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[mariocj89]

Can you add a test that reproduces the issue today and shows it is fixed (to make sure this does not break in the future). The example you have in the issue is good, you can just add it to the testmock.py file.

You also need to add a news entry for this change, something in the lines of:

Add support for calling next with the mock resulting from :func:unittest.mock.mock_open.

Great fix by the way! :)

[tirkarthi]

Please also test __next__ along with next(mock) and also your next calls after exhausting state[0] raises StopIteration so add a test for that too.

I would prefer having these tests grouped under the class for reference

cpython/Lib/unittest/test/testmock/testwith.py

Line 142 in 33e71e0

class TestMockOpen(unittest.TestCase):

This is a code change and would require CLA. Please sign it. Change requires a NEWS entry : https://devguide.python.org/committing/?highlight=news#what-s-new-and-news-entries

Thanks

[Anvil]

@mariocj89 @tirkarthi thank you for your support and your advice. It's been appreciated. I've updated the NEWS and the unittests.

Sorry about the push force, btw, I realized in the mean time i was still using an old user.email to commit and had to amend the commits.

[Anvil]

CLA has been signed, btw.

[tirkarthi]

Personally, grouping the expected data with variables from assert calls reads nice.

line1 = next(f1)
line2 = f1.__next__()
# line 3

self.assertEqual(line1, '1st line\n')
# assert for line 2
# assert for line 3

Also add a test for StopIteration after exhausting f1. You can use

with self.assertRaises(StopIteration):
    next(f1)

[tirkarthi]

Would prefer moving it to TestMockOpen for these tests but would wait for other's opinion on this

[Anvil]

I've applied the changed you've suggested and also added the missing StopIteration test.

[Anvil]

Also, would it be more consistent to test __next__() prior to next() ?

[tirkarthi]

Sorry I implied moving the testmock test to testwith. It seems like two copies.

[Anvil]

Yeah, I've understood that :) but since I see some other already-redundant tests in both files and and since @mariocj89 said testmock and you said testwith.py, I've added both. No offense but I dont know either of you two so I honestly dont who is right. :)

[tirkarthi]

Well, it was just an opinion of mine that to have tests grouped under the class. It's up to the core dev who will be merging this.

[Anvil]

I've added tests to both testmock.py and testwith.py.

[mariocj89]

Sorry, I should have been more specific. This should probably be :func:`next` or double backtick.
I'd suggest :func:`next`

[Anvil]

My bad. Fixed.

[mariocj89]

Looking great IMHO! Thanks, great fix for mock_open.

[asvetlov]

LGTM

[miss-islington]

Thanks @Anvil for the PR 🌮🎉.. I'm working now to backport this PR to: 3.7.
🐍🍒⛏🤖

[asvetlov]

Thanks!

[bedevere-bot]

GH-13521 is a backport of this pull request to the 3.7 branch.

[tirkarthi]

@cjw296 __next__ doesn't seem to exist on python 2 and this might need to be python 3 only code during backport. Perhaps the test could be marked as python 3 only such in the backport test suite.

import mock

mocked_open = mock.mock_open(read_data='one\n')
f1 = mocked_open('a-name')
line1 = f1.__next__()

mock with this patch applied to backport

python /tmp/foo.py
Traceback (most recent call last):
  File "/tmp/foo.py", line 3, in <module>
    mocked_open = mock.mock_open(read_data='one\n')
  File "/Users/karthikeyansingaravelan/stuff/python/27-venv/lib/python2.7/site-packages/mock/mock.py", line 2567, in mock_open
    handle.__next__.side_effect = _next_side_effect
  File "/Users/karthikeyansingaravelan/stuff/python/27-venv/lib/python2.7/site-packages/mock/mock.py", line 695, in __getattr__
    raise AttributeError("Mock object has no attribute %r" % name)
AttributeError: Mock object has no attribute '__next__'

[Anvil]

Thank you everyone.

[asvetlov]

@tirkarthi good point.
I think this is not a reason to reject the PR though :)

[tirkarthi]

@asvetlov Chris is maintaining the mock backport in PyPI that supports Python 2 and hence I was just wanted to make a note of this for Python 2. I am happy with this improving Python 3 :)