title: Pandas: adding column with the length of other column as value
body: <p>I want to add an additional column to an existing dataframe that has the length of the 'seller_name' column as its value. </p>
  
  <p>The output should be like so:</p>
  
  <pre><code>seller_name    name_length
  -------------|-------------
  Rick         |      4
  Hannah       |      6
  </code></pre>
  
  <p>However, I'm having difficulty getting the code right. </p>
  
  <pre><code>df['name_length']  = len(df['seller_name'])
  </code></pre>
  
  <p>just gives me the length of the entire column (6845)
  And </p>
  
  <pre><code>df['nl']  = df[len('seller_name')]
  </code></pre>
  
  <p>Throws a KeyError. </p>
  
  <p>Does anyone know the correct command to achieve my goal?</p>
  
  <p>Many thanks!</p>