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Titlecase, i.e. istitle() and title(), is buggy when the string
includes combining diacritical marks.
>>> u'H\u0301ngh'.istitle()
False
>>> u'H\u0301ngh'.title()
u'H\u0301Ngh'
>>>
The string given already is in titlecase so that the following result
is expected:
>>> u'H\u0301ngh'.istitle()
True
>>> u'H\u0301ngh'.title()
u'H\u0301ngh'
>>>
UTR#21 Case Mappings defines the following algorithm for titlecase
mapping [1]:
For each character C, find the preceding character B. 
 ignore any intervening case-ignorable characters when finding B.
If B exists, and is cased 
 map C to UCD_lower(C)
Otherwise, 
 map C to UCD_title(C)
The class of 'case-ignorable' is defined under [2] and includes
Nonspacing Marks (Mn) as listed in [3]. This includes diacritcal marks
and others. These should not be handled similar to spaces which they
currently are, thus dividing words.
A patch including the above test case is attached.
[1]
http://unicode.org/reports/tr21/tr21-5.html#Case_Conversion_of_Strings
[2] http://unicode.org/reports/tr21/tr21-5.html#Definitions
[3] http://www.fileformat.info/info/unicode/category/Mn/list.htm 

Adding a incomplete patch in need of a function
Py_UNICODE_ISCASEIGNORABLE defining the case-ignorable class.
I don't want to touch capitalize() as I don't fully understand the
semantics, where it is different to title(). It seems though following
UTR#21 not the first character should be uppercased, but the first
character with casing.

Casing algorithms should follow Section 3.13 "Default Case Algorithms"
in the standard itself, not UTR#21.
See
http://www.unicode.org/Public/5.2.0/ucd/DerivedCoreProperties-5.2.0d11.
Unicode 5.2. A nice mail on the Unicode mail list has a bit explanation
to that: http://www.unicode.org/mail-arch/unicode-ml/y2009- 

Implementing full patch solving it the old way (UTR#21).
The correct way for the latest Unicode version would be to implement
the word breaking algorithm described in (UAX#29) [1] first.
[1] http://www.unicode.org/reports/tr29/#Word_Boundaries 

I should add that I didn't include the two header files generated by
Tools/unicode/makeunicodedata.py 

The patch looks good, but it doesn't include the few extra characters
that are also considered case-ignorable:
* U+0027 APOSTROPHE
* U+00AD SOFT HYPHEN (SHY)
* U+2019 RIGHT SINGLE QUOTATION MARK
Could you add those as well ?
Thanks.

> * U+0027 APOSTROPHE
hardcoded (see below)
> * U+00AD SOFT HYPHEN (SHY)
has the "Format (Cf)" property and thus is included automatically
> * U+2019 RIGHT SINGLE QUOTATION MARK
hardcoded (see below)
I hardcoded some characters into Tools/unicode/makeunicodedata.py:
>>> print ' '.join([u':', u'\xb7', u'\u0387', u'\u05f4', u'\u2027',
u'\ufe13', u'\ufe55', u'\uff1a'] + [u"'", u'.', u'\u2018', u'\u2019',
u'\u2024', u'\ufe52', u'\uff07', u'\uff0e'])
: · · ״ ‧ : : : ' . ‘ ’ . . ' .
Those cannot currently be extracted automatically, as neither
DerivedCoreProperties.txt nor the source file for property
"Word_Break(C) = MidLetter or MidNumLet" are provided in the script.
As I said, the patch is only a second best solution, as the correct
path would be implementing the word breaking algorithm as described in
the newest standard. This patch is just an improvement over the current
situation.

Christoph Burgmer wrote:
> 
> Christoph Burgmer <cburgmer@ira.uka.de> added the comment:
> 
>> * U+0027 APOSTROPHE
> hardcoded (see below)
>> * U+00AD SOFT HYPHEN (SHY)
> has the "Format (Cf)" property and thus is included automatically
>> * U+2019 RIGHT SINGLE QUOTATION MARK
> hardcoded (see below)
> 
> I hardcoded some characters into Tools/unicode/makeunicodedata.py:
>>>> print ' '.join([u':', u'\xb7', u'\u0387', u'\u05f4', u'\u2027',
> u'\ufe13', u'\ufe55', u'\uff1a'] + [u"'", u'.', u'\u2018', u'\u2019',
> u'\u2024', u'\ufe52', u'\uff07', u'\uff0e'])
> : · · ״ ‧ : : : ' . ‘ ’ . . ' .
> 
> Those cannot currently be extracted automatically, as neither
> DerivedCoreProperties.txt nor the source file for property
> "Word_Break(C) = MidLetter or MidNumLet" are provided in the script.
As long as those code points are defined somewhere in the Unicode
standard files, that's ok.
It would be good to add a comment explaining the above in the code.
BTW: It's better to use "if (....)" instead of \-line joining. The
parens will automatically have Python do the line joining for you
and it looks better.
> As I said, the patch is only a second best solution, as the correct
> path would be implementing the word breaking algorithm as described in
> the newest standard. This patch is just an improvement over the current
> situation.
We could handle the work-breaking in a separate new method.
For .title(), I think your patch is an improvement and it will
fix most of the cases that issue7008 mentions.

New patch
 - updated comments to reflect needed integration of
DerivedCoreProperties.txt
 - cleaned up if(...) construct
 - updated (from issue7008) and integrated testcase
When applying this patch, run Tools/unicode/makeunicodedata.py to
regenerate the header files.
Note though, that with this patch str and unicode objects will not
behave equally:
>>> s = "This isn't right"
>>> s.title() == unicode(s).title()
False

Referred to this from issue 4610... anyone following this might want to 
look there as well.

So, is it not considered a bug that:
>>> "This isn't right".title()
"This Isn'T Right"
!?!?!?

Jeff Senn wrote:
> 
> Jeff Senn <senn@users.sourceforge.net> added the comment:
> 
> So, is it not considered a bug that:
> 
>>>> "This isn't right".title()
> "This Isn'T Right"
> 
> !?!?!?
That's http://bugs.python.org/issue7008 and is fixed as part of
http://bugs.python.org/issue6412 

@Terry
How is the behavior changed? To me it seems the same to as initially reported.
The results are consistent but nonetheless wrong. It's not about whether your agree with the result, but rather about following the Unicode standard.

Christoph is responding above to a previous version of this message with an erroneous conclusion based on a misreading of his original message.
The proposed patch makes this issue overlap #7008, which had some contentious discussion, so I am adding some people from that to this nosy list so they may opine here. Otherwise starting over:
3.1 has the same bug.
3.1.2
>>> 'H\u0301ngh'.istitle()
False
>>> 'H\u0301ngh'=='H\u0301ngh'.title()
False
>>> 'H\u0301ngh'.title()
'H́Ngh' # in IDLE, the accent is over the H
The problem is that .title() treats the accent that looks like an apostrophe '\u0301' as if it were an apostrophe "'". The latter are documented as forming word boundaries, as in
>>> "De'souza".title()
"De'Souza"
>>> "O'brian".title()
"O'Brian"
Here is the beginning of the 3.1.2 title() doc:
"str.title() 
Return a titlecased version of the string where words start with an uppercase character and the remaining characters are lowercase.
The algorithm uses a simple language-independent definition of a word as groups of consecutive letters. The definition works in many contexts but it means that apostrophes in contractions and possessives form word boundaries, which may not be the desired result:"
That means that
>>> "This Isn'T Right".istitle()
True
is correct as documented.
I interpret the conclusion of #7008, based on Guido's msg93242, as saying that that should be left alone. but I interpret previous messages and the test in unicodeobject.titlecase.3.diff as saying this would become be False. Such a change would badly affect the prior examples where the post ' capital *is* wanted. The is why that change was rejected in #7008. So I think ' should be removed from the current patch. I do not know about the other chars that are hard-coded.
With or without that, there is the issue of whether the current behavior really contradicts the somewhat vague doc and whether change would break enough code that this issue should be treated as a feature change for 3.2 only.
Reading this from msg93265 
"As I said, the patch is only a second best solution, as the correct
path would be implementing the word breaking algorithm as described in
the newest standard. This patch is just an improvement over the current
situation."
makes me wonder whether .title & and .istitle should be left alone until the right solution is implemented.