Question

I want to check during edit form saving process the value of a field verify some constraints

(understand calling a method where I can invalidate the form action)

Answer 1

The field must be defined via schema (not supermodel), otherwise the field isn’t visible in the schema. Once the field is defined in the schema, you may use a decorated function like the following to set a field validator:

@form.validator(field=IMySchema['title'])
def validateTitle(value):
    if value == value.upper():
        raise schema.ValidationError(u"Please don't shout")