Make Pieces immutable and return them from cache

chess/__init__.py

@@ -447,7 +447,7 @@ def between(a: Square, b: Square) -> Bitboard:
 FEN_CASTLING_REGEX = re.compile(r"^(?:-|[KQABCDEFGH]{0,2}[kqabcdefgh]{0,2})\Z")
 
 
-@dataclasses.dataclass
+@dataclasses.dataclass(frozen=True)
 class Piece:
     """A piece with type and color."""
 
@@ -457,6 +457,25 @@ class Piece:
     color: Color
     """The piece color."""
 
+    WHITE_PIECE_OBJECTS: List[Piece] = None  # initialized next
+    BLACK_PIECE_OBJECTS: List[Piece] = None  # initialized next
+
+    @classmethod
+    def from_cache(cls, piece_type: PieceType, color: Color) -> 'Piece':
+        """
+        Fetches a :class:`~chess.Piece` instance from a piece type and color.
+
+        :raises: :exc:`ValueError` if the piece type or color is invalid.
+        """
+
+        if piece_type < 1 or piece_type > 6:
+            raise ValueError("invalid piece type")
+        if color == WHITE:
+            return cls.WHITE_PIECE_OBJECTS[piece_type]
+        elif color == BLACK:
+            return cls.BLACK_PIECE_OBJECTS[piece_type]
+        raise NotImplementedError('invalid color')
+
     def symbol(self) -> str:
         """
         Gets the symbol ``P``, ``N``, ``B``, ``R``, ``Q`` or ``K`` for white
@@ -494,6 +513,19 @@ def from_symbol(cls, symbol: str) -> Piece:
         """
         return cls(PIECE_SYMBOLS.index(symbol.lower()), symbol.isupper())
 
+    @classmethod
+    def initialize_cache(cls):
+        # There is no piece 0, so just fill that slot in so we don't need to do arithmetic later on.
+        cls.WHITE_PIECE_OBJECTS = [None, *(Piece(piece_type, WHITE) for piece_type in PIECE_TYPES)]
+        cls.BLACK_PIECE_OBJECTS = [None, *(Piece(piece_type, BLACK) for piece_type in PIECE_TYPES)]
+        # Sneak in a trampoline to `cls.from_cache()` method.
+        cls.__new__ = piece_from_cache
+
+def piece_from_cache(cls, piece_type: PieceType, color: Color) -> 'Piece':
+    return cls.from_cache(piece_type, color)
+
+Piece.initialize_cache()
+
 
 @dataclasses.dataclass(unsafe_hash=True)
 class Move: