title: Pieces doesn't stack after one loop on my connect4
body: <p>I'm working on a connect4 and I have an issue on the <code>piece slide down</code> part. 
  When I launch the game, the pieces stacked themself only once like that.</p>
  
  <pre><code>000000
  000000
  000000
  000000
  000000
  PP0000
  XX0000
  </code></pre>
  
  <p>But after that, it doesn't stack anymore. If the user chose to put a piece on the first or second row it won't stack the piece over the actual one. I tried to figure out where the issue was coming from but I don't see it and beeing new in python doesn't help a lot. </p>
  
  <pre class="lang-py prettyprint-override"><code>board = []
  
  for x in range(0, 7):
      board.append(["O"] * 6)
  
  def print_board(board):
      for row in board:
          print(" ".join(row))
  
  def user_one(board):
      status = 0
      clm = 0
      first_x = int(input("Enter a Row: "))
      for i in board:
          while i == "0":
              clm += 1
          if board[clm - 1][first_x - 1] == "P":
              board[clm - 2][first_x - 1] = "X"
          else:
              board[clm - 1][first_x - 1] = "X"
      status = 1
      print_board(board)
  
  def user_two(board):
      status = 0
      clm = 0
      second_p = int(input("Enter a Row: "))
      for i in board:
          while i == "0":
              clm += 1
          if board[clm - 1][second_p - 1] == "X":
              board[clm - 2][second_p - 1] = "P"
          else:
              board[clm - 1][second_p - 1] = "P"
      status = 2
      print_board(board)
  
  def launch_game(board):
      while True:
          user_one(board)
          user_two(board)
  
  launch_game(board)
  </code></pre>