title: add a new key value pair to existing key value pair object in python
body: <p>Python beginner here. I have a dictionary with a key and its value is an object (dict) who also has a key value pair.  I want to add a key value pair to the 'child' object. </p>
  
  <p>given: </p>
  
  <pre><code>{"foo" : 
      {"bar" : "bars value"}
  }
  </code></pre>
  
  <p>I want to add:  </p>
  
  <pre><code>{"foo" : 
      {"bar" : "bar value", 
       "baz" : "baz value" 
      }
  }
  </code></pre>
  
  <p>This seems incredibly common but I can't seem to find a good way to do it.  </p>