Fix variable expansion on 'self'

Closes gh-1878 (again)

IPython/core/interactiveshell.py

@@ -2841,9 +2841,11 class InteractiveShell(SingletonConfigurable):
         """
         ns = self.user_ns.copy()
         ns.update(sys._getframe(depth+1).f_locals)
-        ns.pop('self', None)
         try:
-            cmd = formatter.format(cmd, **ns)
+            # We have to use .vformat() here, because 'self' is a valid and common
+            # name, and expanding **ns for .format() would make it collide with
+            # the 'self' argument of the method.
+            cmd = formatter.vformat(cmd, args=[], kwargs=ns)
         except Exception:
             # if formatter couldn't format, just let it go untransformed
             pass

IPython/core/tests/test_interactiveshell.py

@@ -267,7 +267,7 class InteractiveShellTestCase(unittest.TestCase):
                     '  classvar="see me"\n'
                     '  def test(self):\n'
                     '    res = !echo Variable: {self.classvar}\n'
-                    '    return res\n')
+                    '    return res[0]\n')
         nt.assert_in('see me', ip.user_ns['cTest']().test())
     
     def test_bad_var_expand(self):