Incorrect title case?

[John Machin]

On Jan 17, 9:07 am, MRAB <goo... at mrabarnett.plus.com> wrote:
> Python 2.6.1
>
> I've just found that the following 4 Unicode characters/codepoints don't
> behave as I'd expect: Dž (U+01C5), Lj (U+01C8), Nj (U+01CB), Dz (U+01F2).
>
> For example, u"\u01C5".istitle() returns True and
> unicodedata.category(u"\u01C5") returns "Lt", but u"\u01C5".title()
> returns u'\u01C4', which is the uppercase equivalent. Are these mistakes
> in the Unicode database?

Doesn't look like it. AFAICT it's a mistake in Objects/unicodetype.c,
function _PyUnicode_ToTitlecase.

See http://svn.python.org/view/python/trunk/Objects/unicodectype.c?rev=66362&view=markup

The code that says:
    if (ctype->title)
        delta = ctype->title;
    else
	delta = ctype->upper;
should IMHO merely be:
    delta = ctype->title;

A value of zero for ctype->title should be interpreted simply as the
offset to add to the ordinal, as it is in the sibling _PyUnicode_To
(Upper|Lower)case functions. See also Tools/unicode/makeunicodedata.py
which treats upper, lower and title identically when preparing the
tables used by those 3 functions.

AFAICT making that change will fix the problem for those four
characters and not ruin any others.

The error that you noticed occurs as far back as I've looked (2.1) and
also occurs in 3.0.

Cheers,
John