Allow 0 for valuables input

app/means_test/forms/savings.py

@@ -4,7 +4,7 @@
 from app.means_test.forms import BaseMeansTestForm
 from app.means_test.widgets import MoneyInput
 from flask_babel import lazy_gettext as _
-from app.means_test.validators import ValidateIfSession
+from app.means_test.validators import ValidateIfSession, AllowedExceptions
 
 
 class SavingsForm(BaseMeansTestForm):
@@ -44,6 +44,7 @@ class SavingsForm(BaseMeansTestForm):
         validators=[
             ValidateIfSession("has_valuables", True),
             InputRequired(message=_("Enter the total of all valuable items over £500")),
+            AllowedExceptions(0),
             NumberRange(
                 min=50000,
                 message=_("Enter 0 if you have no valuable items worth over £500 each"),

app/means_test/validators.py

@@ -126,3 +126,19 @@ def validate_currency(value: str | None) -> Decimal | None:
             raise ValueError("Enter a valid amount (maximum 2 decimal places)")
 
         return decimal_value
+
+
+class AllowedExceptions:
+    """Allows for additional allowed values that are accepted but would fail subsequent validation.
+    This is typically used for when we want to allow £0 or a value greater than £x.
+    """
+
+    def __init__(self, allowed_values: list[int | str] | int | str):
+        # Convert single value to list for consistency
+        self.allowed_values = (
+            [allowed_values] if not isinstance(allowed_values, list) else allowed_values
+        )
+
+    def __call__(self, form, field):
+        if field.data in self.allowed_values:
+            raise StopValidation()

tests/unit_tests/means_test/test_validators.py

@@ -0,0 +1,41 @@
+from wtforms import Form, IntegerField
+from wtforms.validators import NumberRange
+from app.means_test.validators import AllowedExceptions
+
+
+def test_single_allowed_value():
+    class TestForm(Form):
+        amount = IntegerField(
+            "Amount", validators=[AllowedExceptions(0), NumberRange(min=501)]
+        )
+
+    form = TestForm(amount=0)
+    assert form.validate()
+
+
+def test_list_of_allowed_values():
+    class TestForm(Form):
+        amount = IntegerField(
+            "Amount", validators=[AllowedExceptions([0, 1, 2]), NumberRange(min=501)]
+        )
+
+    # Test each allowed value
+    for value in [0, 1, 2]:
+        form = TestForm(amount=value)
+        assert form.validate()
+
+
+def test_non_allowed_value():
+    class TestForm(Form):
+        amount = IntegerField(
+            "Amount", validators=[AllowedExceptions([0, 1]), NumberRange(min=500)]
+        )
+
+    # Should fail because 3 is neither in allowed values nor >= 500
+    form = TestForm(amount=3)
+    assert form.validate() is False
+    assert "Number must be at least 500." in form.amount.errors
+
+    # Should pass because 500 meets the NumberRange requirement
+    form = TestForm(amount=500)
+    assert form.validate()