Handle substitution errors with no parameters to ValidationError

rest_framework/exceptions.py

@@ -157,7 +157,7 @@ def __init__(self, detail=None, code=None, params=None):
         # 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)):
@@ -166,13 +166,20 @@ def __init__(self, detail=None, code=None, params=None):
                 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, ValueError, TypeError, IndexError, AttributeError):
+            return detail
+
 
 class ParseError(APIException):
     status_code = status.HTTP_400_BAD_REQUEST

tests/test_validation_error.py

@@ -1,3 +1,4 @@
+import pytest
 from django.test import TestCase
 
 from rest_framework import serializers, status
@@ -195,3 +196,92 @@ def test_validation_error_details_validation_errors_nested_list(self):
         assert str(error.detail[1]) == 'Invalid value: 43'
         assert str(error.detail[2]) == 'Invalid value: 44'
         assert str(error.detail[3]) == 'Invalid value: 45'
+
+    def test_validation_error_without_params_string_templating(self):
+        """Ensure that substitutable errors can be emitted without params."""
+
+        # mimic the logic in fields.Field.run_validators by saving the exception
+        # detail into a list which will then be the detail for a new ValidationError.
+        # this should not throw a KeyError or a TypeError even though
+        # the string has a substitutable substring ...
+        errors = []
+        try:
+            raise ValidationError(detail='%(user)s')
+        except ValidationError as exc:
+            errors.extend(exc.detail)
+
+        # ensure it raises the correct exception type as an input to a new ValidationError
+        with pytest.raises(ValidationError):
+            raise ValidationError(errors)
+
+    def test_validation_error_without_params_digit(self):
+        """Ensure that substitutable errors can be emitted with a digit placeholder."""
+
+        # mimic the logic in fields.Field.run_validators by saving the exception
+        # detail into a list which will then be the detail for a new ValidationError.
+        # this should not throw a TypeError on the date format placeholders ...
+        errors = []
+        try:
+            raise ValidationError(detail='%d')
+        except ValidationError as exc:
+            errors.extend(exc.detail)
+
+        # ensure it raises the correct exception type as an input to a new ValidationError
+        with pytest.raises(ValidationError):
+            raise ValidationError(errors)
+
+    def test_validation_error_without_params_date_formatters(self):
+        """Ensure that substitutable errors can be emitted with invalid template placeholders."""
+
+        # mimic the logic in fields.Field.run_validators by saving the exception
+        # detail into a list which will then be the detail for a new ValidationError.
+        # this should not throw a ValueError on the date format placeholders ...
+        errors = []
+        try:
+            raise ValidationError(detail='Expects format %Y-%m-%d %H:%M:%S')
+        except ValidationError as exc:
+            errors.extend(exc.detail)
+
+        # ensure it raises the correct exception type as an input to a new ValidationError
+        with pytest.raises(ValidationError):
+            raise ValidationError(errors)
+
+    def test_validation_error_with_param_that_has_attribute_error(self):
+        """Ensure that substitutable errors can be emitted with a bad string repr."""
+
+        class FooBar:
+            def __str__(self):
+                raise AttributeError("i was poorly coded")
+
+        # mimic the logic in fields.Field.run_validators by saving the exception
+        # detail into a list which will then be the detail for a new ValidationError.
+        # this should not throw a ValueError on the date format placeholders ...
+        errors = []
+        try:
+            raise ValidationError(detail='%s', params=FooBar())
+        except ValidationError as exc:
+            errors.extend(exc.detail)
+
+        # ensure it raises the correct exception type as an input to a new ValidationError
+        with pytest.raises(ValidationError):
+            raise ValidationError(errors)
+
+    def test_validation_error_with_param_that_has_index_error(self):
+        """Ensure that substitutable errors can be emitted with a bad string repr."""
+
+        class FooBar:
+            def __str__(self):
+                raise IndexError("i was poorly coded")
+
+        # mimic the logic in fields.Field.run_validators by saving the exception
+        # detail into a list which will then be the detail for a new ValidationError.
+        # this should not throw a ValueError on the date format placeholders ...
+        errors = []
+        try:
+            raise ValidationError(detail='%s', params=FooBar())
+        except ValidationError as exc:
+            errors.extend(exc.detail)
+
+        # ensure it raises the correct exception type as an input to a new ValidationError
+        with pytest.raises(ValidationError):
+            raise ValidationError(errors)