"@ seems the syntax that everybody can hate equally"
Implementation by Mark Russell, from SF #979728.

after a failed import.

This is the last checkin in the "change import failure semantics" series.

    test_failing_import_sticks -- if an import raises an exception,
        ensure that trying to import it again continues raising exceptions
    test_failing_reload -- if a module loads OK, but a reload raises an
        exception, ensure that the module is still in sys.modules, and
        that its __dict__ reflects as much of the reload attempt as
        succeeded.  That doesn't seem like sane semantics, but it is
        backward-compatible semantics <wink>.

getting left beyind in sys.modules.

PyImport_ReloadModule():  restore the module to sys.modules in error cases.
load_package():  semantic-neutral refactoring from an earlier stab at
                 this patch; giving it a common error exit made the code
                 easier to follow, so retaining that part.
_RemoveModule():  new little utility to delete a key from sys.modules.

in failure cases, incompletely initalized module objects are no longer
left behind in sys.modules.

already correct, so not changed here.

Backport candidate.

Backport candidate.

of no more than 8 elements cannot fail.

listpop():  Take advantage of that its calls to list_resize() and
list_ass_slice() can't fail.  This is assert'ed in a debug build now, but
in an icky way.  That is, you can't say:

	assert(some_call() >= 0);

because then some_call() won't occur at all in a release build.  So it
has to be a big pile of #ifdefs on Py_DEBUG (yuck), or the pleasant:

        status = some_call();
        assert(status >= 0);

But in that case, compilers may whine in a release build, because status
appears unused then.  I'm not certain the ugly trick I used here will
convince all compilers to shut up about status (status is always "used" now,
as the first (ignored) clause in a comma expression).

This fixes 15 spurious test failures on Windows (probably all due to
the test leaving a wrong path in sys.argv[0], which then prevented
regrtest.py from finding the expected-output files for tests running
after test_optparse).

  * add expansion of default values in help text: the string
    "%default" in an option's help string is expanded to str() of
    that option's default value, or "none" if no default value.
  * bug #955889: option default values that happen to be strings are
    now processed in the same way as values from the command line; this
    allows generation of nicer help when using custom types.  Can
    be disabled with parser.set_process_default_values(False).
  * bug #960515: don't crash when generating help for callback
    options that specify 'type', but not 'dest' or 'metavar'.
  * feature #815264: change the default help format for short options
    that take an argument from e.g. "-oARG" to "-o ARG"; add
    set_short_opt_delimiter() and set_long_opt_delimiter() methods to
    HelpFormatter to allow (slight) customization of the formatting.
  * patch #736940: internationalize Optik: all built-in user-
    targeted literal strings are passed through gettext.gettext().  (If
    you want translations (.po files), they're not included with Python
    -- you'll find them in the Optik source distribution from
    http://optik.sourceforge.net/ .)
  * bug #878453: respect $COLUMNS environment variable for
    wrapping help output.
  * feature #988122: expand "%prog" in the 'description' passed
    to OptionParser, just like in the 'usage' and 'version' strings.
    (This is *not* done in the 'description' passed to OptionGroup.)

impossible to remember, so renamed one to something obvious.  Headed
off potential signed-vs-unsigned compiler complaints I introduced by
changing the type of a vrbl to unsigned.  Removed the need for the
tedious explanation about "backward pointer loops" by looping on an
int instead.

result.

list_resize():  Document the intent.  Code is increasingly relying on
subtle aspects of its behavior, and they deserve to be spelled out.

list_ass_slice():  A bit more simplification, by giving it a common
error exit and initializing more values.

Be clearer in comments about what "size" means (# of elements?  # of
bytes?).

While the number of elements in a list slice must fit in an int, there's
no guarantee that the number of bytes occupied by the slice will.  That
malloc() and memmove() take size_t arguments is a hint about that <wink>.
So changed to use size_t where appropriate.

ihigh - ilow should always be >= 0, but we never asserted that.  We do
now.

The loop decref'ing the recycled slice had a subtle insecurity:  C doesn't
guarantee that a pointer one slot *before* an array will compare "less
than" to a pointer within the array (it does guarantee that a pointer
one beyond the end of the array compares as expected).  This was actually
an issue in KSR's C implementation, so isn't purely theoretical.  Python
probably has other "go backwards" loops with a similar glitch.
list_clear() is OK (it marches an integer backwards, not a pointer).

- document the use of the return value for cmd.Cmd.do_*() methods

though I tried to be very careful.  This is a slight simplification, and it
adds a new feature: a small stack-allocated "recycled" array for the cases
when we don't remove too many items.

It allows PyList_SetSlice() to never fail if:
* you are sure that the object is a list; and
* you either do not remove more than 8 items, or clear the list.

This makes a number of other places in the source code correct again -- there
are some places that delete a single item without checking for MemoryErrors
raised by PyList_SetSlice(), or that clear the whole list, and sometimes the
context doesn't allow an error to be propagated.

The invariant checks would break.

  invariants allows the ob_item != NULL check to be replaced with an
  assertion.

* Added assertions to list_init() which document and verify that the
  tp_new slot establishes the invariants.  This may preclude a future
  bug if a custom tp_new slot is written.

  to NULL during the lifetime of the object.

* listobject.c nevertheless did not conform to the other invariants,
  either; fixed.

* listobject.c now uses list_clear() as the obvious internal way to clear
  a list, instead of abusing list_ass_slice() for that.  It makes it easier
  to enforce the invariant about ob_item == NULL.

* listsort() sets allocated to -1 during sort; any mutation will set it
  to a value >= 0, so it is a safe way to detect mutation.  A negative
  value for allocated does not cause a problem elsewhere currently.
  test_sort.py has a new test for this fix.

* listsort() leak: if items were added to the list during the sort, AND if
  these items had a __del__ that puts still more stuff into the list,
  then this more stuff (and the PyObject** array to hold them) were
  overridden at the end of listsort() and never released.

* Use plain wording in docs for id().
* Use normal quotation marks instead of single quotes in the description.

mutation during list.sort() used to rely on that listobject.c always
NULL'ed ob_item when ob_size fell to 0.  That's no longer true, so the
test for list mutation during a sort is no longer reliable.  Changed the
test to rely instead on that listobject.c now never NULLs-out ob_item
after (if ever) ob_item gets a non-NULL value.  This new assumption is
also documented now, as a required invariant in listobject.h.

The new assumption allowed some real simplification to some of the
hairier code in listsort(), so is a Good Thing on that count.

Backport candidate.

invariants they must satisfy.

the size_t nbytes, and passed nbytes to malloc, so it was confusing to
effectively recompute the same thing from scratch in the memset call.

Py_RETURN_FALSE and Py_RETURN_TRUE.