moving info between function in class
<p>I want to write a function that holds all the strings that the user insert to argument <code>plays</code> in function <code>move()</code>, to the string <code>undo_str</code> in the function <code>undo()</code>.
what am I missing here?</p>

<pre><code>class Sokoban:
        def __init__(self, board):
                self.board = board.copy()

        def move(self, plays):
             ............

        def undo(self):
                undo_str=""
                undo_str=undo_str[:]+plays
                self.undo=undo_str[:-1]
</code></pre>
