Doubled backslash in repr() method for unicode

[Cito]

BPO	1459029
Nosy	@Cito, @hyeshik
Files	uni.diff: patch for test

^{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/hyeshik'
closed_at = <Date 2006-03-28.07:39:56.000>
created_at = <Date 2006-03-27.02:54:37.000>
labels = ['expert-unicode']
title = 'Doubled backslash in repr() method for unicode'
updated_at = <Date 2006-03-28.07:39:56.000>
user = 'https://github.com/Cito'

bugs.python.org fields:

activity = <Date 2006-03-28.07:39:56.000>
actor = 'anthonybaxter'
assignee = 'hyeshik.chang'
closed = True
closed_date = None
closer = None
components = ['Unicode']
creation = <Date 2006-03-27.02:54:37.000>
creator = 'cito'
dependencies = []
files = ['1933']
hgrepos = []
issue_num = 1459029
keywords = []
message_count = 9.0
messages = ['27885', '27886', '27887', '27888', '27889', '27890', '27891', '27892', '27893']
nosy_count = 4.0
nosy_names = ['nnorwitz', 'anthonybaxter', 'cito', 'hyeshik.chang']
pr_nums = []
priority = 'high'
resolution = 'fixed'
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue1459029'
versions = ['Python 2.4']

[Cito]

Here is an issue that caused Kid templates (used by
Turbogears) to malfunction in Python 2.4.3c1.

The problem shows up with the following code:

class s1:
    def __repr__(self):
        return '\\n'

class s2:
    def __repr__(self):
        return u'\\n'

print repr(s1()), repr(s2())

I get the following results:

Python 2.3.5: \n \n
Python 2.4.2: \n \n
Python 2.4.3c1: \n \\n

In the output for Python 2.4.3c1, the backslash in the
representation of class2 appears doubled. This did not
happen in earlier Python versions and seems to be a bug.

My vague guess is that the issue may have crept in with
an attempted fix of Bug bpo-1379994.

-- Christoph

[anthonybaxter]

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Confirmed - it's also broken in the trunk, and backing out
the patch for http://www.python.org/sf/1379994 (r41728)
fixes the problem. Perky, you checked this in - can you look
at this soon, please? I don't want to release 2.4.3 until
it's fixed, but I also want to get 2.4.3 out this week.

Thanks for the bug report!

[nnorwitz]

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Attached a patch for the test case to be added with fix.

[hyeshik]

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Looking the C code, unicode_repr is doing correct.
But the inconsistency came from PyObject_Repr.
This change made it which is intended:

------------------------------------------------------------------------
r16198 | effbot | 2000-07-09 02:43:32 +0900 (�, 09 7 2000)
| 6 lines

• changed __repr__ to use "unicode escape" encoding for unicode
  strings, instead of the default encoding.
  (see "minidom" thread for discussion, and also patch bpo-100706)

[hyeshik]

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Found it!:
http://mail.python.org/pipermail/python-dev/2000-July/005353.html
But their intention had never applied before 2.4.3.
What problem would be if we change PyObject_Repr to use the
default encoding not unicode-escape? (revert r16198)

[anthonybaxter]

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I'm confused how a checkin from 5+ years ago broke a change
from 3 months ago?

Or am I misunderstanding you?

[hyeshik]

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Because unicode-escape codec didn't escape \,
PyObject_Repr(u'\\') bypassed backslashes. But Martin and
Fredrik made PyObject_Repr to use unicode-escape codec for
unicode repr-returns 5 years ago. So by fixing
unicode-escape codec, their intention could be applied for
the first time, 3 months ago.

[nnorwitz]

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We need to retain the old behaviour, but also fix the bug.
How can we do that?

[anthonybaxter]

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Ok. After talking to perky, I reverted the fix for 1379994
on the release24-maint branch, and reverted /F's ancient
change on the trunk. This seemed the best combination of
practicality and purity. Fix will be in 2.4.3 final.

Thanks for the bug report. Man, unicode and repr is a twisty
ball of horrors.