New test for sorting sanity. Note that this will fail in earlier Pythons,

in the stability tests.

Bizarre:  this takes 11x longer to run if and only if test_longexp is
run before it, on my box.  The bigger REPS is in test_longexp, the
slower this gets.  What happens on your box?  It's not gc on my box
(which is good, because gc isn't a plausible candidate here).

The slowdown is massive in the parts of test_sort that implicitly
invoke a new-style class's __lt__ or __cmp__ methods.  If I boost
REPS large enough in test_longexp, even the test_sort tests on an array
of size 64 visibly c-r-a-w-l.  The relative slowdown is even worse in
a debug build.  And if I reduce REPS in test_longexp, the slowdown in
test_sort goes away.

test_longexp does do horrid things to Win98's management of user
address space, but I thought I had made that a whole lot better a month
or so ago (by overallocating aggressively in the parser).

Lib/test/test_sort.py

@@ -0,0 +1,124 @@
+from test.test_support import verbose
+import random
+
+nerrors = 0
+
+def check(tag, expected, raw, compare=None):
+    global nerrors
+
+    if verbose:
+        print "    checking", tag
+
+    orig = raw[:]   # save input in case of error
+    if compare:
+        raw.sort(compare)
+    else:
+        raw.sort()
+
+    if len(expected) != len(raw):
+        print "error in", tag
+        print "length mismatch;", len(expected), len(raw)
+        print expected
+        print orig
+        print raw
+        nerrors += 1
+        return
+
+    for i, good in enumerate(expected):
+        maybe = raw[i]
+        if good is not maybe:
+            print "error in", tag
+            print "out of order at index", i, good, maybe
+            print expected
+            print orig
+            print raw
+            nerrors += 1
+            return
+
+# Try a variety of sizes at and around powers of 2, and at powers of 10.
+sizes = [0]
+for power in range(1, 10):
+    n = 2 ** power
+    sizes.extend(range(n-1, n+2))
+sizes.extend([10, 100, 1000])
+
+class Complains(object):
+    maybe_complain = True
+
+    def __init__(self, i):
+        self.i = i
+
+    def __lt__(self, other):
+        if Complains.maybe_complain and random.random() < 0.001:
+            if verbose:
+                print "        complaining at", self, other
+            raise RuntimeError
+        return self.i < other.i
+
+    def __repr__(self):
+        return "Complains(%d)" % self.i
+
+class Stable(object):
+    maybe_complain = True
+
+    def __init__(self, key, i):
+        self.key = key
+        self.index = i
+
+    def __cmp__(self, other):
+        return cmp(self.key, other.key)
+
+    def __repr__(self):
+        return "Stable(%d, %d)" % (self.key, self.index)
+
+for n in sizes:
+    x = range(n)
+    if verbose:
+        print "Testing size", n
+
+    s = x[:]
+    check("identity", x, s)
+
+    s = x[:]
+    s.reverse()
+    check("reversed", x, s)
+
+    s = x[:]
+    random.shuffle(s)
+    check("random permutation", x, s)
+
+    y = x[:]
+    y.reverse()
+    s = x[:]
+    check("reversed via function", y, s, lambda a, b: cmp(b, a))
+
+    if verbose:
+        print "    Checking against an insane comparison function."
+        print "        If the implementation isn't careful, this may segfault."
+    s = x[:]
+    s.sort(lambda a, b:  int(random.random() * 3) - 1)
+    check("an insane function left some permutation", x, s)
+
+    x = [Complains(i) for i in x]
+    s = x[:]
+    random.shuffle(s)
+    Complains.maybe_complain = True
+    it_complained = False
+    try:
+        s.sort()
+    except RuntimeError:
+        it_complained = True
+    if it_complained:
+        Complains.maybe_complain = False
+        check("exception during sort left some permutation", x, s)
+
+    s = [Stable(random.randrange(10), i) for i in xrange(n)]
+    augmented = [(e, e.index) for e in s]
+    augmented.sort()    # forced stable because ties broken by index
+    x = [e for e, i in augmented] # a stable sort of s
+    check("stability", x, s)
+
+if nerrors:
+    print "Test failed", nerrors
+elif verbose:
+    print "Test passed -- no errors."