Design Snake Game — Python

# Time:  O(1) per move
# Space: O(s), s is the current length of the snake.

from collections import deque

class SnakeGame(object):

    def __init__(self, width,height,food):
        """
        Initialize your data structure here.
        @param width - screen width
        @param height - screen height
        @param food - A list of food positions
        E.g food = [[1,1], [1,0]] means the first food is positioned at [1,1], the second is at [1,0].
        :type width: int
        :type height: int
        :type food: List[List[int]]
        """
        self.__width = width
        self.__height = height
        self.__score = 0
        self.__f = 0
        self.__food = food
        self.__snake = deque([(0, 0)])
        self.__direction = {"U": (-1, 0), "L": (0, -1), "R": (0, 1), "D": (1, 0)}
        self.__lookup = {(0, 0)}

    def move(self, direction):
        """
        Moves the snake.
        @param direction - 'U' = Up, 'L' = Left, 'R' = Right, 'D' = Down
        @return The game's score after the move. Return -1 if game over.
        Game over when snake crosses the screen boundary or bites its body.
        :type direction: str
        :rtype: int
        """
        def valid(x, y):
            return 0 <= x < self.__height and \
                   0 <= y < self.__width and \
                   (x, y) not in self.__lookup
        d = self.__direction[direction]
        x, y = self.__snake[-1][0] + d[0], self.__snake[-1][1] + d[1]
        self.__lookup.remove(self.__snake[0])
        tail = self.__snake.popleft()
        if not valid(x, y):
            return -1
        elif self.__f != len(self.__food) and (self.__food[self.__f][0], self.__food[self.__f][1]) == (x, y):
            self.__score += 1
            self.__f += 1
            self.__snake.appendleft(tail)
            self.__lookup.add(tail)
        self.__snake.append((x, y))
        self.__lookup.add((x, y))
        return self.__score