def validate(self, value):
        """
        Applies the validation criteria.
        Returns value, new value, or None if invalid.

        Overload this in derived classes.
        """
        try:
            # trap blank fields here
            if not self.blank or value:
                float(value)
            return value
        except ValueError:
            return None
Applies the validation criteria.
        Returns value, new value, or None if invalid.

        Overload this in derived classes.