assertEqual doesn't display newline differences quite well

[pitrou]

BPO	12038
Nosy	@terryjreedy, @gpshead, @pitrou, @ezio-melotti, @voidspace, @iritkatriel
Files	issue-12038.patch: Print a more human readable assert message when two strings don't match.

^{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/voidspace'
closed_at = None
created_at = <Date 2011-05-09.16:36:50.398>
labels = ['type-bug', 'library', '3.11']
title = "assertEqual doesn't display newline differences quite well"
updated_at = <Date 2021-12-09.21:45:16.085>
user = 'https://github.com/pitrou'

bugs.python.org fields:

activity = <Date 2021-12-09.21:45:16.085>
actor = 'iritkatriel'
assignee = 'michael.foord'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2011-05-09.16:36:50.398>
creator = 'pitrou'
dependencies = []
files = ['22486']
hgrepos = []
issue_num = 12038
keywords = ['patch']
message_count = 7.0
messages = ['135608', '135657', '135953', '136007', '136079', '139190', '408160']
nosy_count = 7.0
nosy_names = ['terry.reedy', 'gregory.p.smith', 'pitrou', 'ezio.melotti', 'michael.foord', 'mouad', 'iritkatriel']
pr_nums = []
priority = 'low'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue12038'
versions = ['Python 3.11']

[pitrou]

>>> tc.assertEqual("x\n" * 40 + "\n", "x\n" * 40 + "\r\n")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/antoine/cpython/default/Lib/unittest/case.py", line 662, in assertEqual
    assertion_func(first, second, msg=msg)
  File "/home/antoine/cpython/default/Lib/unittest/case.py", line 1041, in assertMultiLineEqual
    self.fail(self._formatMessage(msg, standardMsg))
  File "/home/antoine/cpython/default/Lib/unittest/case.py", line 531, in fail
    raise self.failureException(msg)
AssertionError: 'x\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx [truncated]... != 'x\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx [truncated]...
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It isn't obvious when looking at this printout that the last newline differed.

[voidspace]

Yup, I agree that's pretty sucky. Any suggestions for a fix?

[terryjreedy]

A possible fix is to condense the output by omitting stuff in the center rather than as the end:
"x\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx...x\nx\nx\nx\nx\nx\nx\nx\nx\nx\n"
"x\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx...x\nx\nx\nx\nx\nx\nx\nx\nx\nx\r\n"
would be clear.

[gpshead]

We should ensure that the first differing character in the string is always included in what is displayed as a diff. if we're going to shorten a string we should elide something that matches.

[voidspace]

Yep, ensuring the truncated repr includes at least the *first* difference sounds like the right approach.

[mouad]

I took on consideration the two suggestion that was in the issue and create this patch that basically show only until the last character that differ and truncate the rest.

I just included one test case, other test case if they should be included (and if my solution got accepted :)), should be more *complex* and check using Regex the correctness of the assertion message, but here is the output that we can see when applying the patch:

>> tc.assertEqual("x\n" * 40 + "\n", "x\n" * 40 + "\r\n")

'x\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\n...nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\n\n [truncated]... != 'x\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\n...\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\n\r [truncated]...
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>> tc.assertEqual('aaaa', 'abbb')

AssertionError: 'aa [truncated]... != 'ab [truncated]...

• aaaa
  + abbb

>> tc.assertEqual('aaaa', 'bbbb')

AssertionError: 'a [truncated]... != 'b [truncated]...

• aaaa
  + bbbb

>> tc.assertEqual("x\n" * 80 + "\n", "x\n" * 80 + "\r")

AssertionError: 'x\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\n...nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\n\n [truncated]... != 'x\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\n...nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\nx\n\r [truncated]...

[iritkatriel]

Reproduced on 3.11.