Backport PR #10896: Ensure that MimeBundleFormatter always returns a 2-tuple

IPython/core/formatters.py

@@ -217,7 +217,7 b' def catch_format_error(method, self, *args, **kwargs):'
         r = method(self, *args, **kwargs)
     except NotImplementedError:
         # don't warn on NotImplementedErrors
-        return None
+        return self._check_return(None, args[0])
     except Exception:
         exc_info = sys.exc_info()
         ip = get_ipython()
@@ -225,7 +225,7 b' def catch_format_error(method, self, *args, **kwargs):'
             ip.showtraceback(exc_info)
         else:
             traceback.print_exception(*exc_info)
-        return None
+        return self._check_return(None, args[0])
     return self._check_return(r, args[0])
 
 

IPython/core/tests/test_formatters.py

@@ -521,3 +521,13 b' def test_repr_mime_meta():'
             'height': 10,
         }
     })
+
+def test_repr_mime_failure():
+    class BadReprMime(object):
+        def _repr_mimebundle_(self, include=None, exclude=None):
+            raise RuntimeError
+
+    f = get_ipython().display_formatter
+    obj = BadReprMime()
+    d, md = f.format(obj)
+    nt.assert_in('text/plain', d)