title: Count Most Frequent Values for Key in Dictionary
body: <p>I have a Pandas Dataframe where the column 'items' is a dictionary and shows per transaction which products have been bought:</p>
  <pre><code>data = {'price':[40, 15, 10, 2],
      'items': [&quot;{'product': 'Product1', 'quantity': 4, 'product': 'Product2', 'quantity': 1}&quot;, &quot;{'product': 'Product2', 'quantity': 1, 'product': 'Product3', 'quantity': 1,'product': 'Product1', 'quantity': 1}&quot;, &quot;{'product': 'Product1', 'quantity': 4}&quot;, &quot;{'product': 'Product3', 'quantity': 1, 'product': 'Product1', 'quantity': 1}&quot;]
      }
  df = pd.DataFrame (data, columns = ['price', 'items'])
  </code></pre>
  <p><a href="https://i.stack.imgur.com/hvJXN.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/hvJXN.png" alt="enter image description here" /></a></p>
  <p>I want to find out which products have been bought most. In this case the result should look like:</p>
  <pre><code>Product1: 4
  Product2: 2
  </code></pre>
  <p>How can I count the most frequent values of the key 'product' within the column 'items'?</p>