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I'm sometimes using an array.array with format character "u" as a writable backing store for buffers shared with platform APIs that access buffers of UCS2 values. This works fine in python 3.2 and earlier with a ucs2 build of python, but no longer works with python 3.3 because the "u" character explicitly selects a UCS4 representation in that version.
An example of how I use this is using PyObjC on MacOSX, for example:
b = array.array('u', "hello world")
s = CFStringCreateMutableWithExternalCharactersNoCopy( 
 None, b, len(b), len(b), kCFAllocatorNull)
"s" now refers to a mutable Objective-C string that uses "b" as its backing store.
It would be nice if there were a format code that would allow me to do this with Python 3.3, for example b = array.array("U", ...)
(BTW. I'm sorry if this is a duplicate, searching for "array.array" on the tracker results in a lot of hits, most of which have nothing to do with the array module)

See also #13072 and the discussion starting at:
http://mail.python.org/pipermail/python-dev/2012-March/117390.html
I think the priority should be "high", since the current behavior
doesn't preserve the status quo. Also, PEP-3118 suggests 'u' for
UCS2 and 'w' for UCS4.

Hmm, obviously the discussion starts here:
http://mail.python.org/pipermail/python-dev/2012-March/117376.html 

This one should be fixed by #13072. Could you check again?

As Stefan noted, so long as Py_UNICODE is 16 bits in the Mac OS X builds, then this should now be back to the 3.2 behaviour.

It's not back to the 3.2 behavior. In 3.3, Py_UNICODE is always equal to wchar_t, which is a 4-byte type on Darwin. However, CFString is based on UniChar, which is a 2-byte type.
That this worked in 3.2 was by accident - it would work only in "narrow" builds. Python's configure in 3.2 and before wouldn't default to using wchar_t on Darwin since it didn't consider wchar_t "usable", which in turn happened because wchar_t is signed on Darwin, but Py_UNICODE was understood to be unsigned.
Since it's too late to add an 'U' code to 3.3, as a work-around, you would have to use a 'H' array, and initialize it with map(ord, the_string)).
Chances are good that a proper UCS-2 array code gets added to 3.4.

Py_UNICODE is an typedef for wchar_t and that type is 4 bytes long:
>>> a.tobytes()
b'h\x00\x00\x00e\x00\x00\x00l\x00\x00\x00l\x00\x00\x00o\x00\x00\x00 \x00\x00\x00w\x00\x00\x00o\x00\x00\x00r\x00\x00\x00l\x00\x00\x00d\x00\x00\x00'
>>> a = array.array('u', 'bar')
>>> a.tobytes()
b'b\x00\x00\x00a\x00\x00\x00r\x00\x00\x00'
>>> len(a.tobytes())
12
>>> 
This is with a checkout that was created yesterday.
The issue is not resolved, there now is no way to easily create a UCS2 buffer; while there was in earlier releases of Python (with the default narrow build)