How to do simple value check with error message in Deform/Colander

I'm implementing a simple 'tick开发者_Python百科 to agree to terms and conditions box' in Deform/Colander.

So, I simply want to check that the box is checked and have an error message saying 'You must agree to T&C'.

I understand that I can use:

colander.OneOf([True]) 

to ensure the box is ticked. However, OneOf does not allow for a custom error message. What would the correct way to do this be?

---

Use a custom validator:

def t_and_c_validator(node, value):
    if not value:
        raise Invalid(node, 'You must agree to the T&C')

class MySchema(colander.Schema):
    t_and_c = colander.SchemaNode(
                  colander.Boolean(),
                  description='Terms and Conditions',
                  widget=deform.widget.CheckboxWidget(),
                  title='Terms and Conditions',
                  validator=t_and_c_validator,
                  )