title: Adding keys with empty values and then adding values in a nested format in python
body: <p>I am trying to create and add elements to a dictionary in the following format, </p>
  
  <pre><code>{{"source1": { "destination11": ["datetime1", "datetime2", ....]}
              { "destination12": ["datetime3", "datetime4",....]}
              ........................................
  }
  {"source2": { "destination21": ["datetime5", "datetime6", ....]}
              { "destination22": ["datetime7", "datetime8",....]}
              .......................................
  }
  .........................................}
  </code></pre>
  
  <p>All the keys and values are variables which I am getting from other modules.
   I have created an empty dictionary
  call_record=[{}]
  To add the "source1", "source2" as keys I tried,</p>
  
  <pre><code>call_record.append({source1 :})
  </code></pre>
  
  <p>Now i can't add a value to this key yet because i will be adding it  in the next lines, so I need to create this key with empty value and then add values as I get them from the next modules. But, this line doesn't create a key with empty value.</p>
  
  <p>Besides, to add "destination11", "destination12"ets, I tried,</p>
  
  <pre><code>call_record[i].append(destination11) 
  </code></pre>
  
  <p>But, this doesn't add the destinations as the values of the source key.</p>
  
  <p>I have to add the datetimes, after I have added the destinations. Then I have to dump this dictionary in a json file.</p>