use real Warnings for formatter errors

adds FormatterWarning warning class, available for suppression

IPython/core/formatters.py

@@ -183,6 +183,9 b' class DisplayFormatter(Configurable):'
 # Formatters for specific format types (text, html, svg, etc.)
 #-----------------------------------------------------------------------------
 
+class FormatterWarning(UserWarning):
+    """Warning class for errors in formatters"""
+
 @decorator
 def warn_format_error(method, self, *args, **kwargs):
     """decorator for warning on failed format call"""
@@ -192,16 +195,19 b' def warn_format_error(method, self, *args, **kwargs):'
         # don't warn on NotImplementedErrors
         return None
     except Exception as e:
-        warn("Exception in %s formatter: %s" % (self.format_type, e))
+        warnings.warn("Exception in %s formatter: %s" % (self.format_type, e),
+            FormatterWarning,
+        )
         return None
     if r is None or isinstance(r, self._return_type) or \
         (isinstance(r, tuple) and r and isinstance(r[0], self._return_type)):
         return r
     else:
-        warn("%s formatter returned invalid type %s (expected %s) for object: %s" % (
-            self.format_type, type(r), self._return_type, pretty._safe_repr(args[0])
-        ))
-    
+        warnings.warn(
+            "%s formatter returned invalid type %s (expected %s) for object: %s" % \
+            (self.format_type, type(r), self._return_type, pretty._safe_repr(args[0])),
+            FormatterWarning
+        )
 
 
 class FormatterABC(with_metaclass(abc.ABCMeta, object)):

IPython/core/tests/test_formatters.py

@@ -240,7 +240,7 b' def test_warn_error_method():'
     with capture_output() as captured:
         result = f(bad)
     nt.assert_is(result, None)
-    nt.assert_in("WARNING", captured.stderr)
+    nt.assert_in("FormatterWarning", captured.stderr)
     nt.assert_in("text/html", captured.stderr)
     nt.assert_in("zero", captured.stderr)
 
@@ -254,7 +254,7 b' def test_nowarn_notimplemented():'
     with capture_output() as captured:
         result = f(h)
     nt.assert_is(result, None)
-    nt.assert_not_in("WARNING", captured.stderr)
+    nt.assert_not_in("FormatterWarning", captured.stderr)
 
 def test_warn_error_for_type():
     f = HTMLFormatter()
@@ -262,7 +262,7 b' def test_warn_error_for_type():'
     with capture_output() as captured:
         result = f(5)
     nt.assert_is(result, None)
-    nt.assert_in("WARNING", captured.stderr)
+    nt.assert_in("FormatterWarning", captured.stderr)
     nt.assert_in("text/html", captured.stderr)
     nt.assert_in("name_error", captured.stderr)
 
@@ -275,7 +275,7 b' def test_warn_error_pretty_method():'
     with capture_output() as captured:
         result = f(bad)
     nt.assert_is(result, None)
-    nt.assert_in("WARNING", captured.stderr)
+    nt.assert_in("FormatterWarning", captured.stderr)
     nt.assert_in("text/plain", captured.stderr)
     nt.assert_in("argument", captured.stderr)