from __future__ import annotations

import re
from dataclasses import dataclass

# UK postcode format, matched against the canonical (no-space, upper) value.
# A pure structural check — it does not confirm the postcode actually exists
# (that would need a network lookup, which a value object must not do).
_UK_POSTCODE_RE = re.compile(r"[A-Z]{1,2}\d[A-Z\d]?\d[A-Z]{2}")


@dataclass(frozen=True)
class Postcode:
    value: str

    def __post_init__(self) -> None:
        # Frozen dataclass: bypass the descriptor with object.__setattr__.
        object.__setattr__(self, "value", "".join(self.value.split()).upper())

    def __str__(self) -> str:
        return self.value

    def is_valid(self) -> bool:
        """Whether the canonical value is a well-formed UK postcode (format
        only — no existence check, so it is pure and offline)."""
        return _UK_POSTCODE_RE.fullmatch(self.value) is not None