Bonus exercise (AtariGo) about 2020-2021 HOT 2 OPEN

import random


def place_stone(colour, black, white):
    if colour == "black":
        place = random.choice(possible_move("black", black, white))
        return place
    elif colour == "white":
        place = random.choice(possible_move("white", black, white))
        return place


def possible_move(colour, black, white): # check the free position near the opposite color
    if colour == "black":
        movement_list = []
        for position in white:
            list_single_move = single_stone(position, black, white)
            movement_list.extend(list_single_move)
        return movement_list
    elif colour == "white":
        movement_list = []
        for position in black:
            list_single_move = single_stone(position, black, white)
            movement_list.extend(list_single_move)
        return movement_list


def single_stone(position, black, white): # check the free position near the stone and return a list of free position
    list_movement = []
    if (position[0] - 1, position[1]) not in black and (position[0] - 1, position[1]) not in white and position[0] - 1 <= 6:
        list_movement.append((position[0] - 1, position[1]))
    if (position[0] + 1, position[1]) not in black and (position[0] + 1, position[1]) not in white and position[0] + 1 <= 6:
        list_movement.append((position[0] + 1, position[1]))
    if (position[0], position[1] - 1) not in black and (position[0], position[1] - 1) not in white and position[1] - 1 <= 6:
        list_movement.append((position[0], position[1] - 1))
    if (position[0], position[1] + 1) not in black and (position[0], position[1] + 1) not in white and position[1] + 1 <= 6:
        list_movement.append((position[0], position[1] + 1))
    return list_movement


print(place_stone("white", {(1, 5), (1, 4), (2, 6)}, {(2, 5), (2, 4)}))

def place_stone(color,black,white):
  
  if len(black & white)==0:     # Check that black and white sets don't have common elements
  
    range_x=range(7)            # Maximum number of columns
    range_y=range(7)            # Maximum number of rows
    occupied=black.union(white) # Defines a set with all the occupied cells regardless of the color
    
    expanding=list()  # Prepares the list of moves that expand the liberties
    limiting=list()   # Prepares the list of moves that limit opponent's liberties 
    others=list()     # Prepres the list of all other positions
    
    # Gets rid of the black/white dialectics and switches to current/opponent
    if color=="white":
      current=white
      opponent=black
    else:
      current=black
      opponent=white
    
    # Cycles all the positions of the board:
    for x in range_x:
      for y in range_y:
        
        if ((x,y) not in occupied): # if position is empty
          
          if check_neighbors(x,y,opponent): # if position limits opponent liberties
            limiting.append((x,y))
          if check_neighbors(x,y,current):  # if position expands current color liberties
            expanding.append((x,y))
          if ((x,y) not in limiting) and ((x,y) not in expanding): # if position has no neighbor stones
            others.append((x,y))
            
    bestpos=set(limiting) & set(expanding) # Finds the positons limiting the opponent and expanding the current group
    
    if len(bestpos)>0:
      return(list(bestpos)[0]) # Returns the first result that expand your liberty and limits the opponent
          
    if len(limiting)>0:
      return(limiting[0])      # Else returns the first result that limits the opponent
              
    if len(expanding)>0:
      return expanding[0]      # Else returns the first result that expands your liberties
            
    if len(others)>0:
      return (others[0])       # Else returns the first free position 
          
    return "No position available"  # Else no move can be done
    
  else:   
    return "Each point admits only one stone!"

# This ancillary function only returns if x,y position is adjacent to something in checking set
def check_neighbors(x,y,checking):
  return ((x-1,y) in checking) or ((x+1,y) in checking) or ((x,y-1) in checking) or ((x,y+1) in checking)


# TEST CASES  
  
color="white"
black={(2,2),(2,3),(2,4),(2,5)}
white={(2,1),(3,2),(3,3)}
print (place_stone(color,black,white)) # Answers (3,4)

color="black"
black={(4,3),(4,4),(4,5),(5,3),(5,5),(6,3),(6,5)}
white={(5,4)}
print (place_stone(color,black,white)) # Answers (6,4)... My boy!

color="black"
black={(4,3),(4,4),(4,5),(5,3),(5,5),(6,3),(6,5)}
white={(4,3)}
print (place_stone(color,black,white)) # Returns an error