Fix representation of sql.Placeholder

Fix also Placeholder tests, including an error which made an assert
always true, which made us miss the regression in #1291.

lib/sql.py

@@ -438,7 +438,10 @@ def name(self):
         return self._wrapped
 
     def __repr__(self):
-        return f"Placeholder({self._wrapped if self._wrapped is not None else ''!r})"
+        if self._wrapped is None:
+            return f"{self.__class__.__name__}()"
+        else:
+            return f"{self.__class__.__name__}({self._wrapped!r})"
 
     def as_string(self, context):
         if self._wrapped is not None:

tests/test_sql.py

@@ -373,29 +373,25 @@ def test_name(self):
         self.assertEqual(sql.Placeholder('foo').name, 'foo')
 
     def test_repr(self):
-        self.assert_(str(sql.Placeholder()), 'Placeholder()')
-        self.assert_(repr(sql.Placeholder()), 'Placeholder()')
-        self.assert_(sql.Placeholder().as_string(self.conn), '%s')
+        self.assertEqual(str(sql.Placeholder()), 'Placeholder()')
+        self.assertEqual(repr(sql.Placeholder()), 'Placeholder()')
+        self.assertEqual(sql.Placeholder().as_string(self.conn), '%s')
 
     def test_repr_name(self):
-        self.assert_(str(sql.Placeholder('foo')), "Placeholder('foo')")
-        self.assert_(repr(sql.Placeholder('foo')), "Placeholder('foo')")
-        self.assert_(sql.Placeholder('foo').as_string(self.conn), '%(foo)s')
+        self.assertEqual(str(sql.Placeholder('foo')), "Placeholder('foo')")
+        self.assertEqual(repr(sql.Placeholder('foo')), "Placeholder('foo')")
+        self.assertEqual(sql.Placeholder('foo').as_string(self.conn), '%(foo)s')
 
     def test_bad_name(self):
         self.assertRaises(ValueError, sql.Placeholder, ')')
 
     def test_eq(self):
-        self.assert_(sql.Placeholder('foo') == sql.Placeholder('foo'))
-        self.assert_(sql.Placeholder('foo') != sql.Placeholder('bar'))
-        self.assert_(sql.Placeholder('foo') != 'foo')
-        self.assert_(sql.Placeholder() == sql.Placeholder())
-        self.assert_(sql.Placeholder('foo') != sql.Placeholder())
-        self.assert_(sql.Placeholder('foo') != sql.Literal('foo'))
-
-    def test_as_string(self):
-        self.assertEqual(sql.Placeholder().as_string(self.conn), "%s")
-        self.assertEqual(sql.Placeholder('foo').as_string(self.conn), "%(foo)s")
+        self.assertEqual(sql.Placeholder('foo'), sql.Placeholder('foo'))
+        self.assertNotEqual(sql.Placeholder('foo'), sql.Placeholder('bar'))
+        self.assertNotEqual(sql.Placeholder('foo'), 'foo')
+        self.assertEqual(sql.Placeholder(), sql.Placeholder())
+        self.assertNotEqual(sql.Placeholder('foo'), sql.Placeholder())
+        self.assertNotEqual(sql.Placeholder('foo'), sql.Literal('foo'))
 
 
 class ValuesTest(ConnectingTestCase):