Read this today:

http://mortoray.com/2013/11/27/the-string-type-is-broken/

In it the author talks about how the 'ffl' ligature breaks some string processing.  He claimed that Python 3 doesn't uppercase it correctly--well, it does.  However I discovered that it doesn't reverse it properly.

    x = b'ba\xef\xac\x84e'.decode('utf-8') # "baffle", where "ffl" is a ligature
    print(x) # prints "baffle", with the ligature
    print(x.upper())  # prints "BAFFLE", no ligature, which is fine
    print("".join(reversed(x))) # prints "efflab"

Shouldn't that last line print "elffab"?

If this gets marked as "wontfix" I wouldn't complain.  Just wondering what the Right Thing is to do here.

There is no ligature for "lff", just "ffl". Ligatures are treated as one char. I guess Python would have to grow a str.reverse() method to handle ligatures and combining chars correctly.

At work I ran into the issue with ligatures and combining chars multiple times in medieval and early modern age scripts. Eventually I started to normalize all incoming data to NFKC. That solves most of the issues.

s = b'ba\xef\xac\x84e'.decode('utf-8')
>>> print("".join(reversed(s)))
efflab
>>> print("".join(reversed(unicodedata.normalize("NFKC", s))))
elffab

A proper str.reverse function must deal with more extra cases. For example there are special rules for the Old German long s (ſ) and the round s (s). A round s may only occur at the end of a syllable. Hebrew has a special variant of several characters if the character is placed at the end of a word (HEBREW LETTER PE / HEBREW LETTER FINAL PE).

A simple reversed(s) can never deal with all the complicated rules.

Python implements the Unicode standards. Except if Python failed to implement the standard correctly, the author should complain to the Unicode Consortium directly!
http://www.unicode.org/contacts.html

Example of data for the "ffl" character, U+FB04:

FB04;LATIN SMALL LIGATURE FFL;Ll;0;L;<compat> 0066 0066 006C;;;;N;;;;;

http://www.unicode.org/Public/6.0.0/ucd/UnicodeData.txt

(I'm unable to decode these raw data :-))

I don't understand the purpose of using reversed(). Don't use it to display a text backward. Handling bidirectional text requires more complex tools to display such text. See for example the pango library:
https://developer.gnome.org/pango/stable/pango-Bidirectional-Text.html

I don't see anything wrong with Python here, it just implements the Unicode standards, so I'm closing the issue as invalid.