title: How to add new value in existing key in dictionary(without losing the previous values)?
body: <p>I have this dictionary filled with keys (each key is a tuple) and value None for each key.
  I want to add values (one at a time) within a for loop to the keys, without overwriting values</p>
  
  <p>This code has an error message " AttributeError: 'NoneType' object has no attribute 'append' "</p>
  
  <pre><code>dictionary1={}
  dictionary1.update( {(0,0) : None} )
  
  listOfTuples=[(0,0),(1,0),(9,0)]
  for x in range(3):
      dictionary1[(0,0)].append(listOfTuples(x))
  </code></pre>
  
  <p>The result i want is the dictionary1 to be like:</p>
  
  <pre><code>{ (0,0) : [(0,0),(1,0),(9,0)] }
  </code></pre>
  
  <p>But i want it to be filled in the for loop one tuple at a time</p>