Marc-Andre Lemburg <report@bugs.python.org> wrote
   on Tue, 16 Aug 2011 12:11:22 -0000: 

> The reasoning behind e.g. "ISSURROGATE" is that those names originate
> from and are consistent with the already existing ISLOWER/ISUPPER/ISTITLE
> macros which in return stem from the C APIs of the same names
> (see unicodeobject.h for reference).

I eventually figured that part out in the larger context.  
Makes sense looked at that way.

> Regarding low/high vs. lead/trail: The Unicode database uses the terms
> low/high and we do in Python as well, so let's stick with those.

Yes, those are their block assignments,  Block=High_Surrogates and Block=Low_Surrogates.
I just thought I should mention that in the time since those were invented (which cannot
be changed), after using them in real code for some years, their lingo seems to have 
evolved away from those initial names and toward lead/trail as less confusing.

> What I don't understand is why those macros should be declared
> private to Python (with the leading underscore). They are quite
> useful for extensions implementing codecs or other transformations
> as well.

I was wondering about that myself.  Beyond there being a lot fewer of those
private macros in the Python *.h files, they also seem to be of rather different
character than the iswhatever() macros:

    $ perl -nle '/^\s*#\s*define\s+(_Py_[\p{Lu}_]+)\b/ and print $1' *.h | sort -dfu | fmt -160
    _Py_ANNOTATE_BARRIER_DESTROY _Py_ANNOTATE_BARRIER_INIT _Py_ANNOTATE_BARRIER_WAIT_AFTER _Py_ANNOTATE_BARRIER_WAIT_BEFORE _Py_ANNOTATE_BENIGN_RACE
    _Py_ANNOTATE_BENIGN_RACE_SIZED _Py_ANNOTATE_BENIGN_RACE_STATIC _Py_ANNOTATE_CONDVAR_LOCK_WAIT _Py_ANNOTATE_CONDVAR_SIGNAL _Py_ANNOTATE_CONDVAR_SIGNAL_ALL
    _Py_ANNOTATE_CONDVAR_WAIT _Py_ANNOTATE_ENABLE_RACE_DETECTION _Py_ANNOTATE_EXPECT_RACE _Py_ANNOTATE_FLUSH_STATE _Py_ANNOTATE_HAPPENS_AFTER
    _Py_ANNOTATE_HAPPENS_BEFORE _Py_ANNOTATE_IGNORE_READS_AND_WRITES_BEGIN _Py_ANNOTATE_IGNORE_READS_AND_WRITES_END _Py_ANNOTATE_IGNORE_READS_BEGIN
    _Py_ANNOTATE_IGNORE_READS_END _Py_ANNOTATE_IGNORE_SYNC_BEGIN _Py_ANNOTATE_IGNORE_SYNC_END _Py_ANNOTATE_IGNORE_WRITES_BEGIN _Py_ANNOTATE_IGNORE_WRITES_END
    _Py_ANNOTATE_MUTEX_IS_USED_AS_CONDVAR _Py_ANNOTATE_NEW_MEMORY _Py_ANNOTATE_NO_OP _Py_ANNOTATE_PCQ_CREATE _Py_ANNOTATE_PCQ_DESTROY _Py_ANNOTATE_PCQ_GET
    _Py_ANNOTATE_PCQ_PUT _Py_ANNOTATE_PUBLISH_MEMORY_RANGE _Py_ANNOTATE_PURE_HAPPENS_BEFORE_MUTEX _Py_ANNOTATE_RWLOCK_ACQUIRED _Py_ANNOTATE_RWLOCK_CREATE
    _Py_ANNOTATE_RWLOCK_DESTROY _Py_ANNOTATE_RWLOCK_RELEASED _Py_ANNOTATE_SWAP_MEMORY_RANGE _Py_ANNOTATE_THREAD_NAME _Py_ANNOTATE_TRACE_MEMORY
    _Py_ANNOTATE_UNPROTECTED_READ _Py_ANNOTATE_UNPUBLISH_MEMORY_RANGE _Py_AS_GC _Py_CHECK_REFCNT _Py_COUNT_ALLOCS_COMMA _Py_DEC_REFTOTAL _Py_DEC_TPFREES
    _Py_INC_REFTOTAL _Py_INC_TPALLOCS _Py_INC_TPFREES _Py_PARSE_PID _Py_REF_DEBUG_COMMA _Py_SET_EDOM_FOR_NAN

> BTW: I think the other issues mentioned in the discussion are more
> important to get right, than the names of those macros.

Yup.  Just paint it red. :)

--tom