def postcode_in_state(self, state_abbr: Optional[str] = None) -> str:
        """
        Returns a random postcode within the provided province abbreviation.

        :param state_abbr: A province abbreviation.
        :returns: A random postcode within the provided province abbreviation.
        """
        if state_abbr is None:
            state_abbr = self.random_element(self.provinces_abbr)

        if state_abbr in self.provinces_abbr:
            postcode = str(
                self.generator.random.randint(
                    self.provinces_postcode[state_abbr][0], self.provinces_postcode[state_abbr][1]
                )
            )

            # zero left pad up until desired length (length is 6)
            target_postcode_len = 6
            current_postcode_len = len(postcode)
            if current_postcode_len < target_postcode_len:
                pad = target_postcode_len - current_postcode_len
                postcode = f"{'0' * pad}{postcode}"

            return postcode

        raise ValueError("Province Abbreviation not found in list")

        Returns a random postcode within the provided province abbreviation.

        :param state_abbr: A province abbreviation.
        :returns: A random postcode within the provided province abbreviation.
        
faker/providers/address/vi_VN/__init__.py