title: How to create key or append an element to key?
body: <p>I have an empty dictionary. Name: <code>dict_x</code>
  It is to have keys of which values are lists.</p>
  <p>From a separate iteration, I obtain a key (ex: <code>key_123</code>), and an item (a tuple) to place in the list of <code>dict_x</code>'s value <code>key_123</code>.</p>
  <p>If this key already exists, I want to append this item.
  If this key does not exist, I want to create it with an empty list and then append to it or just create it with a tuple in it.</p>
  <p>In future when again this key comes up, since it exists, I want the value to be appended again.</p>
  <p>My code consists of this:</p>
  <blockquote>
  <p>Get key and value.</p>
  <p>See if <strong>NOT</strong> key exists in <code>dict_x</code>.</p>
  <p>and if not create it: <code>dict_x[key] == []</code></p>
  <p>Afterwards: <code>dict_x[key].append(value)</code></p>
  </blockquote>
  <p>Is this the way to do it? Shall I try to use <code>try/except</code> blocks?</p>