def _validate(self, value):
        """
        Predicate used to determine if a computed value is valid, True, or
        not, False.
        """
        if value is None and not self.nullable:
            self.ctx.errors.invalid('not nullable')
            return False
        return True
def _validate ( self , value ) : if value is None and not self . nullable : self . ctx . errors . invalid ( 'not nullable' ) return False return True
Predicate used to determine if a computed value is valid, True, or
        not, False.