doctest 'fancy diff' formats incorrectly strip trailing whitespace

[bitdancer]

BPO	24746
Nosy	@tim-one, @orsenthil, @vstinner, @ned-deily, @bitdancer, @CuriousLearner, @pablogsal, @miss-islington
PRs	bpo-24746: Avoid stripping trailing whitespace in doctest fancy diff #10639 bpo-24746: Avoid stripping trailing whitespace in doctest fancy diff #10639 [3.7] bpo-24746: Avoid stripping trailing whitespace in doctest fancy diff (GH-10639) #11476 [3.7] bpo-24746: Avoid stripping trailing whitespace in doctest fancy diff (GH-10639) #11476 [3.7] bpo-24746: Avoid stripping trailing whitespace in doctest fancy diff (GH-10639) #11476 [3.6] bpo-24746: Avoid stripping trailing whitespace in doctest fancy diff (GH-10639) #11477 [3.6] bpo-24746: Avoid stripping trailing whitespace in doctest fancy diff (GH-10639) #11477 [3.6] bpo-24746: Avoid stripping trailing whitespace in doctest fancy diff (GH-10639) #11477 [2.7] bpo-24746: Avoid stripping trailing whitespace in doctest fancy diff #11482 [2.7] bpo-24746: Avoid stripping trailing whitespace in doctest fancy diff #11482 Revert "bpo-24746: Avoid stripping trailing whitespace in doctest fan… #11496 Revert "bpo-24746: Avoid stripping trailing whitespace in doctest fan… #11496 Revert "bpo-24746: Avoid stripping trailing whitespace in doctest fan… #11496 [3.7] Revert "bpo-24746: Avoid stripping trailing whitespace in doctest fan… #11498 [3.7] Revert "bpo-24746: Avoid stripping trailing whitespace in doctest fan… #11498 [3.7] Revert "bpo-24746: Avoid stripping trailing whitespace in doctest fan… #11498 [3.6] Revert "bpo-24746: Avoid stripping trailing whitespace in doctest fan… #11499 [3.6] Revert "bpo-24746: Avoid stripping trailing whitespace in doctest fan… #11499 [3.6] Revert "bpo-24746: Avoid stripping trailing whitespace in doctest fan… #11499 bpo-24746: Fix doctest failures when running the testsuite with -R #11501 bpo-24746: Fix doctest failures when running the testsuite with -R #11501 bpo-24746: Fix doctest failures when running the testsuite with -R #11501 [3.7] bpo-24746: Fix doctest failures when running the testsuite with -R (GH-11501) #11505 [3.7] bpo-24746: Fix doctest failures when running the testsuite with -R (GH-11501) #11505 [3.7] bpo-24746: Fix doctest failures when running the testsuite with -R (GH-11501) #11505 [3.6] - "Revert bpo-24746: Avoid stripping trailing whitespace in doctest fancy diff (GH-10639)" #11509 [3.6] - "Revert bpo-24746: Avoid stripping trailing whitespace in doctest fancy diff (GH-10639)" #11509 [3.6] - "Revert bpo-24746: Avoid stripping trailing whitespace in doctest fancy diff (GH-10639)" #11509 [2.7] bpo-24746: Fix doctest failures when running the testsuite with -R #11512 [2.7] bpo-24746: Fix doctest failures when running the testsuite with -R #11512 [2.7] bpo-24746: Fix doctest failures when running the testsuite with -R #11512 [2.7] bpo-24746: Fix doctest failures when running the testsuite with -R #11512
Files	doctest_fancy_diff_trailing_whitespace.diff issue24746.patch

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/orsenthil'
closed_at = <Date 2019-01-11.01:18:33.446>
created_at = <Date 2015-07-29.01:26:52.557>
labels = ['3.7', '3.8', 'type-bug', 'library', 'easy']
title = "doctest 'fancy diff' formats incorrectly strip trailing whitespace"
updated_at = <Date 2019-01-11.13:58:34.268>
user = 'https://github.com/bitdancer'

bugs.python.org fields:

activity = <Date 2019-01-11.13:58:34.268>
actor = 'vstinner'
assignee = 'orsenthil'
closed = True
closed_date = <Date 2019-01-11.01:18:33.446>
closer = 'orsenthil'
components = ['Library (Lib)']
creation = <Date 2015-07-29.01:26:52.557>
creator = 'r.david.murray'
dependencies = []
files = ['40049', '40191']
hgrepos = []
issue_num = 24746
keywords = ['patch', 'easy']
message_count = 26.0
messages = ['247552', '248695', '248701', '248726', '248727', '330214', '330220', '330240', '333319', '333320', '333322', '333341', '333360', '333361', '333362', '333363', '333367', '333372', '333382', '333388', '333390', '333392', '333415', '333417', '333431', '333468']
nosy_count = 9.0
nosy_names = ['tim.peters', 'orsenthil', 'vstinner', 'ned.deily', 'r.david.murray', 'jairotrad', 'CuriousLearner', 'pablogsal', 'miss-islington']
pr_nums = ['10639', '10639', '11476', '11476', '11476', '11477', '11477', '11477', '11482', '11482', '11496', '11496', '11496', '11498', '11498', '11498', '11499', '11499', '11499', '11501', '11501', '11501', '11505', '11505', '11505', '11509', '11509', '11509', '11512', '11512', '11512', '11512']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue24746'
versions = ['Python 2.7', 'Python 3.7', 'Python 3.8']

[bitdancer]

I got a doctest failure that when I wrote the output to a file showed two exactly identical lines reported as being different. Turning off the fancy diff, I could see trailing whitespace on one of the lines. It turns out that when a fancy diff is requested, doctest explicitly goes through and strips trailing whitespace from the diff lines returned by difflib. This seems to me to be obviously incorrect. There is no clue in the changelog why this was done...this goes back to a massive refactoring of doctest that was done for python 2.4, and the fancy diff was introduced at that point, complete with this strange behavior.

I tried to write a test for this but couldn't get it working in the time I was willing to devote to this (I've switched to NDIFF format, which shows the whitespace error even when the actual whitespace is stripped). Perhaps testing this via doctest isn't the best idea anyway, since it will be far to easy for the trailing whitespace in the test to get accidentally stripped. I've attached my work as a diff for reference if someone wants to work on this.

[jairotrad]

I have patched this as explained by David. Also the tests are working. Another test broke because a missing trailing space. I fixed that too.

This is my first patch :D

[bitdancer]

Unfortunately I was reminded a few days ago that there is a commit hook that prevents patches containing trailing whitespace from being committed to the repository. So using doctest to test this isn't going to work.

The alternatives are to write a unit test, or to figure out how to write the doctest such there is no trailing whitespace...which would mean capturing the output of doctest.DoctTestRunner instead of letting it go the console, and then comparing it to an explicitly embedded string.

[jairotrad]

I can write a unittest for this, where should I write it? a new test file?

[bitdancer]

I believe you can put it in the test_doctest file, and call it using the appropriate runner from test.support from the test_main function. I haven't tried it though.

[CuriousLearner]

Unfortunately I was reminded a few days ago that there is a commit hook that prevents patches containing trailing whitespace from being committed to the repository.

Is this still the case?

I tried to figure out writing the test case, but I can't wrap my head around showing the trailing whitespace without putting it anywhere in the code.

Also, can you have a look at the PR, please?

[tim-one]

To include trailing whitespace on a line in a doctest, _don't_ use raw strings. Use a regular string, and include add a (one or more) trailing \x20 instead of a space (for example). For example:

r"""
>>> print("a ")
a 
"""

where there's a space at the end of the output line is no good. It's visually impossible to tell what's intended, and the commit hook should reject it. But this works fine (a regular string and an escape code):

"""
>>> print("a ")
a\x20
"""

[CuriousLearner]

Thanks for your suggestion, Tim.

I've fixed the patch and it is ready for review at #10639

[orsenthil]

New changeset cbb1645 by Senthil Kumaran (Sanyam Khurana) in branch 'master':
bpo-24746: Avoid stripping trailing whitespace in doctest fancy diff (bpo-10639)
cbb1645