Validate missing values. Raise a :exc:`ValidationError` if
        `value` should be considered missing.

def _validate_missing(self, value):
        """Validate missing values. Raise a :exc:`ValidationError` if
        `value` should be considered missing.
        """
        if value is missing_:
            if hasattr(self, 'required') and self.required:
                self.fail('required')
        if value is None:
            if hasattr(self, 'allow_none') and self.allow_none is not True:
                self.fail('null')