ENH: Make the set pretty-printer sort the elements if possible, similar to dict keys.

IPython/lib/pretty.py

@@ -554,7 +554,14 b' def _set_pprinter_factory(start, end, basetype):'
         else:
             step = len(start)
             p.begin_group(step, start)
-            for idx, x in enumerate(obj):
+            # Like dictionary keys, we will try to sort the items.
+            items = list(obj)
+            try:
+                items.sort()
+            except Exception:
+                # Sometimes the items don't sort.
+                pass
+            for idx, x in enumerate(items):
                 if idx:
                     p.text(',')
                     p.breakable()

IPython/lib/tests/test_pretty.py

@@ -89,10 +89,10 b' def test_sets():'
     """
     Test that set and frozenset use Python 3 formatting.
     """
-    objects = [set(), frozenset(), set([1]), frozenset([1]), set([1,2]),
-        frozenset([1,2])]
+    objects = [set(), frozenset(), set([1]), frozenset([1]), set([1, 2]),
+        frozenset([1, 2]), set([-1, -2, -3])]
     expected = ['set()', 'frozenset()', '{1}', 'frozenset({1})', '{1, 2}',
-        'frozenset({1, 2})']
+        'frozenset({1, 2})', '{-3, -2, -1}']
     for obj, expected_output in zip(objects, expected):
         got_output = pretty.pretty(obj)
         yield nt.assert_equal, got_output, expected_output