Why does my grid have a period as a placeholder?

from cards import Card
import random


class Game:
    def __init__(self):
        self.size = 4
        self.card_options = ['Add', 'Boo', 'Cat', 'Dev', 'Egg', 'Far', 'Gum', 'Hut']
        self.columns = ['A', 'B', 'C', 'D']
        self.cards = []
        self.locations = []
        for column in self.columns:
            for num in range(1, self.size + 1):
                self.locations.append(f'{column}{num}')
    def set_cards(self):
        used_locations = []
        for word in self.card_options:
            for i in range(2):
                available_locations = set(self.locations) - set(used_locations)
                random_location = random.choice(list(available_locations))
                used_locations.append(random_location)
                card = Card(word, random_location)
                self.cards.append(card)

    def create_row(self, num):
      row = []
      for column in self.columns:
          for card in self.cards:
              if card.location == f'{column}{num}':
                  if card.matched:
                      row.append(str(card))
                  else:
                      row.append('.  ')
      return row

    def create_grid(self):
        # / A / B / C / D /
        header = ' |  ' + '  |  '.join(self.columns) + '  |'
        print(header)
        for row in range(1, self.size + 1):
          print_row = f'{row}| '
          get_row = self.create_row(row)
          print_row += ' | '.join(get_row) + ' |'
          print(print_row)



# methods
    # create our grid
    # check for matches
    # check game won
    # run the game

# dunder main
if __name__ == '__main__':
    game = Game()
    game.set_cards()
    game.cards[0].matched = True
    game.cards[1].matched = True
    game.cards[2].matched = True
    game.cards[3].matched = True
    game.create_grid()
    # call start game