Indexer with two keys in python
<p>I'm newbie with python. I want to write a class with two keys as indexer. also need to be able to use them inside of class like this:</p>

<pre><code>a = Cartesian(-10,-10,10,10) # Cartesian is the name of my class
a[-5][-1]=10
</code></pre>

<p>and in the Cartesian class:</p>

<pre><code>def fill(self,value):
   self[x][y] = x*y-value
</code></pre>

<p>I try with</p>

<pre><code>def __getitem__(self,x,y):
  return self.data[x-self.dx][y-self.dy]
</code></pre>

<p>but doesn't work.</p>
