check return types of formatters

and show error messages when formatters fail

IPython/core/formatters.py

@@ -33,6 +33,7 b' from IPython.external.decorator import decorator'
 # Our own imports
 from IPython.config.configurable import Configurable
 from IPython.lib import pretty
+from IPython.utils import io
 from IPython.utils.traitlets import (
     Bool, Dict, Integer, Unicode, CUnicode, ObjectName, List,
 )
@@ -182,15 +183,21 b' class DisplayFormatter(Configurable):'
 # Formatters for specific format types (text, html, svg, etc.)
 #-----------------------------------------------------------------------------
 
-
 @decorator
 def warn_format_error(method, self, *args, **kwargs):
     """decorator for warning on failed format call"""
     try:
-        return method(self, *args, **kwargs)
+        r = method(self, *args, **kwargs)
     except Exception as e:
         warn("Exception in %s formatter: %s" % (self.format_type, e))
         return None
+    if r is None or isinstance(r, self._return_type):
+        return r
+    else:
+        warn("%s formatter returned invalid type %s for object: %s" % (
+            self.format_type, type(r), pretty._safe_repr(obj)
+        ))
+    
 
 
 class FormatterABC(with_metaclass(abc.ABCMeta, object)):
@@ -262,6 +269,7 b' class BaseFormatter(Configurable):'
     """
 
     format_type = Unicode('text/plain')
+    _return_type = string_types
 
     enabled = Bool(True, config=True)
 
@@ -278,7 +286,7 b' class BaseFormatter(Configurable):'
     # The deferred-import type-specific printers.
     # Map (modulename, classname) pairs to the format functions.
     deferred_printers = Dict(config=True)
-
+    
     @warn_format_error
     def __call__(self, obj):
         """Compute the format for an object."""
@@ -679,6 +687,8 b' class PNGFormatter(BaseFormatter):'
     format_type = Unicode('image/png')
 
     print_method = ObjectName('_repr_png_')
+    
+    _return_type = bytes
 
 
 class JPEGFormatter(BaseFormatter):
@@ -696,6 +706,8 b' class JPEGFormatter(BaseFormatter):'
 
     print_method = ObjectName('_repr_jpeg_')
 
+    _return_type = bytes
+
 
 class LatexFormatter(BaseFormatter):
     """A LaTeX formatter.