Handle date placeholders in error messages too.

Signed-off-by: James Tanner <tanner.jc@gmail.com>

rest_framework/exceptions.py

@@ -149,7 +149,7 @@ class ValidationError(APIException):
     status_code = status.HTTP_400_BAD_REQUEST
     default_detail = _('Invalid input.')
     default_code = 'invalid'
-    default_params = SafeReplacerDict()
+    default_params = {}
 
     def __init__(self, detail=None, code=None, params=None):
         if detail is None:
@@ -162,7 +162,7 @@ class ValidationError(APIException):
         # For validation failures, we may collect many errors together,
         # so the details should always be coerced to a list if not already.
         if isinstance(detail, str):
-            detail = [detail % params]
+            detail = [self.template_detail(detail, params)]
         elif isinstance(detail, ValidationError):
             detail = detail.detail
         elif isinstance(detail, (list, tuple)):
@@ -171,13 +171,22 @@ class ValidationError(APIException):
                 if isinstance(detail_item, ValidationError):
                     final_detail += detail_item.detail
                 else:
-                    final_detail += [detail_item % params if isinstance(detail_item, str) else detail_item]
+                    final_detail += [self.template_detail(detail_item, params) if isinstance(detail_item, str) else detail_item]
             detail = final_detail
         elif not isinstance(detail, dict) and not isinstance(detail, list):
             detail = [detail]
 
         self.detail = _get_error_details(detail, code)
 
+    def template_detail(self, detail, params):
+        """Handle error messages with templates and placeholders."""
+        try:
+            return detail % params
+        except KeyError:
+            return detail
+        except ValueError:
+            return detail
+
 
 class ParseError(APIException):
     status_code = status.HTTP_400_BAD_REQUEST

tests/test_validation_error.py

@@ -164,7 +164,6 @@ class TestValidationErrorWithDjangoStyle(TestCase):
             params={'value3': '44'}
         )
         assert isinstance(error.detail, list)
-        import pdb; pdb.set_trace()
         assert len(error.detail) == 3
         assert str(error.detail[0]) == 'Invalid value: 42'
         assert str(error.detail[1]) == 'Invalid value: 43'