title: Error in writing Python dictionary to a file
body: <p>This is a simplified version of my code:</p>
  
  <pre><code>test={}
  for i in range(10):
     for j in range(15):
       test[i,j]=i*j
  with open('boo.json', 'w') as f:
      json.dump(test, f)
  </code></pre>
  
  <p>and I got the following error:</p>
  
  <blockquote>
    <p>raise TypeError("key " + repr(key) + " is not a string")</p>
  </blockquote>
  
  <p>Any solution?</p>