Validators correctly raise exceptions

CHANGES.rst

@@ -69,6 +69,9 @@ Unreleased
 -   form.errors is not cached and will update if an error is appended to a field after access. (:pr:`568`)
 -   :class:`~wtforms.validators.NumberRange` correctly handle *not a number*
     values. (:pr:`505`, :pr:`548`)
+-   :class:`ValueError` raised by a validator are handled like regular exceptions. validators
+    need to raise :class:`~wtforms.validators.ValidationError` or
+    :class:`~wtforms.validators.StopValidation` to make a validation fail (:pr:`445`, :pr:`551`).
 
 
 Version 2.2.1

src/wtforms/fields/core.py

@@ -11,7 +11,7 @@
 from wtforms.compat import izip, text_type
 from wtforms.i18n import DummyTranslations
 from wtforms.utils import unset_value
-from wtforms.validators import StopValidation
+from wtforms.validators import StopValidation, ValidationError
 
 __all__ = (
     "BooleanField",
@@ -246,7 +246,7 @@ def validate(self, form, extra_validators=()):
             if e.args and e.args[0]:
                 self.errors.append(e.args[0])
             stop_validation = True
-        except ValueError as e:
+        except ValidationError as e:
             self.errors.append(e.args[0])
 
         # Run validators
@@ -257,7 +257,7 @@ def validate(self, form, extra_validators=()):
         # Call post_validate
         try:
             self.post_validate(form, stop_validation)
-        except ValueError as e:
+        except ValidationError as e:
             self.errors.append(e.args[0])
 
         return len(self.errors) == 0
@@ -277,7 +277,7 @@ def _run_validation_chain(self, form, validators):
                 if e.args and e.args[0]:
                     self.errors.append(e.args[0])
                 return True
-            except ValueError as e:
+            except ValidationError as e:
                 self.errors.append(e.args[0])
 
         return False
@@ -553,7 +553,7 @@ def pre_validate(self, form):
                 if self.data == v:
                     break
             else:
-                raise ValueError(self.gettext("Not a valid choice"))
+                raise ValidationError(self.gettext("Not a valid choice"))
 
 
 class SelectMultipleField(SelectField):
@@ -591,7 +591,7 @@ def pre_validate(self, form):
             values = list(c[0] for c in self.choices)
             for d in self.data:
                 if d not in values:
-                    raise ValueError(
+                    raise ValidationError(
                         self.gettext("'%(value)s' is not a valid choice for this field")
                         % dict(value=d)
                     )

tests/test_fields.py

@@ -293,9 +293,9 @@ def pre_validate(self, form):
 
     def post_validate(self, form, stopped):
         if self.data == "p":
-            raise ValueError("Post")
+            raise validators.ValidationError("Post")
         elif stopped and self.data == "stop-post":
-            raise ValueError("Post-stopped")
+            raise validators.ValidationError("Post-stopped")
 
 
 class TestPrePostValidation:
@@ -1002,9 +1002,9 @@ def test_min_max_entries(self):
     def test_validators(self):
         def validator(form, field):
             if field.data and field.data[0] == "fail":
-                raise ValueError("fail")
+                raise validators.ValidationError("fail")
             elif len(field.data) > 2:
-                raise ValueError("too many")
+                raise validators.ValidationError("too many")
 
         F = make_form(a=FieldList(self.t, validators=[validator]))
 

tests/test_validators.py

@@ -3,7 +3,7 @@
 import pytest
 import re
 
-from wtforms import Label
+from wtforms import Label, Form, StringField
 from wtforms.compat import text_type
 from wtforms.validators import (
     StopValidation,
@@ -670,3 +670,17 @@ def test_regexp_message(dummy_form, dummy_field, grab_error_message):
     validator = regexp("^a", message="foo")
     dummy_field.data = "f"
     assert grab_error_message(validator, dummy_form, dummy_field) == "foo"
+
+
+@pytest.mark.parametrize("exc", [IndexError, ZeroDivisionError, ValueError])
+def test_raise_exceptions(exc):
+    def validate(form, field):
+        raise exc
+
+    class F(Form):
+        field = StringField(validators=[validate])
+
+    f = F()
+
+    with pytest.raises(exc):
+        f.validate()