Fix dispatching in the pretty printing module.

Search for the best method to use for pretty printing an object
now no longer prefers any registered printer in type_printers for
the class or any of the base classes over a _repr_pretty_ method
defined in the class or any of its base classes.

Instead the mro is walked, looking for both registered printers
and _repr_pretty_ methods, so that the inheritance hierarchy will
be taken into account.

IPython/lib/pretty.py

@@ -336,21 +336,25 b' class RepresentationPrinter(PrettyPrinter):'
                 pass
             else:
                 return printer(obj, self, cycle)
-            # Next look for type_printers.
+            # Next walk the mro and check for either:
+            #   1) a registered printer
+            #   2) a _repr_pretty_ method
             for cls in _get_mro(obj_class):
                 if cls in self.type_pprinters:
+                    # printer registered in self.type_pprinters
                     return self.type_pprinters[cls](obj, self, cycle)
                 else:
+                    # deferred printer
                     printer = self._in_deferred_types(cls)
                     if printer is not None:
                         return printer(obj, self, cycle)
-            # Finally look for special method names.
-            if hasattr(obj_class, '_repr_pretty_'):
-                # Some objects automatically create any requested
-                # attribute. Try to ignore most of them by checking for
-                # callability.
-                if callable(obj_class._repr_pretty_):
-                    return obj_class._repr_pretty_(obj, self, cycle)
+                    else:
+                        # Finally look for special method names.
+                        # Some objects automatically create any requested
+                        # attribute. Try to ignore most of them by checking for
+                        # callability.
+                        if callable(getattr(obj_class, '_repr_pretty_')):
+                            return obj_class._repr_pretty_(obj, self, cycle)
             return _default_pprint(obj, self, cycle)
         finally:
             self.end_group()

IPython/lib/tests/test_pretty.py

@@ -40,6 +40,11 b' class MyList(object):'
                     p.pretty(child)
 
 
+class MyDict(dict):
+    def _repr_pretty_(self, p, cycle):
+        p.text("MyDict(...)")
+
+
 def test_indentation():
     """Test correct indentation in groups"""
     count = 40
@@ -47,3 +52,14 b' def test_indentation():'
     expectedoutput = "MyList(\n" + ",\n".join("   %d" % i for i in range(count)) + ")"
 
     nt.assert_equals(gotoutput, expectedoutput)
+
+
+def test_dispatch():
+    """
+    Test correct dispatching: The _repr_pretty_ method for MyDict
+    must be found before the registered printer for dict.
+    """
+    gotoutput = pretty.pretty(MyDict())
+    expectedoutput = "MyDict(...)"
+
+    nt.assert_equals(gotoutput, expectedoutput)