title: Python how to update dict without overwriting other values
body: <p>suppose I have this:</p>
  
  <pre><code>config = {
      "a": {
          "hello": 1,
          "goodbye": 2,
      }
  }
  </code></pre>
  
  <p>and I want to update <code>["a"]["hello"]</code> to 10 like this:</p>
  
  <pre><code>update = {
      "a": {
          "hello": 10
      }
  }
  
  config.update(update)
  </code></pre>
  
  <p>at this point config is now:</p>
  
  <pre><code>config = {
      "a": {
          "hello": 10
      }
  }
  </code></pre>
  
  <p>How can I update one dict with another dict without overwriting other values/subdicts?</p>