title: How to used np.where to return a list of tuples with all positions in a 2D numpy array equal to zero?
body: <p>I am learning about numpy and as an exercise I have to create a function possibilities that has as an input a numpy 2D array with integers and must return a list of tuples where the values are zeros. I've been told that numpy has a function where that can help. I read the docs but couldn't understand it at least not for this task so I had to do it with for loops like this:</p>
  
  <pre><code>def possibilities(board):
      not_occupied = []
      for i in range(len(board)):
          for j in range(len(board[0])):
              if board[i][j] == 0:
                  not_occupied.append((i,j))
      return not_occupied
  </code></pre>
  
  <p>Board is somthing like this:</p>
  
  <pre><code>board = [[1,2,0],[0,0,1],[2,0,1]]
  </code></pre>
  
  <p>How I could use numpy where to do that instead?</p>