title: How do I add a string value to DataFrame?
body: <pre><code>string = 'cool'
  df = pd.DataFrame(columns=['string_values'])
  </code></pre>
  
  <p>Append</p>
  
  <pre><code>df.append(string)
  </code></pre>
  
  <p>I get this error when I try to append it into df. (Is it only for numerical data?)</p>
  
  <pre><code>cannot concatenate object of type "&lt;class 'str'&gt;"; only pd.Series, pd.DataFrame, and pd.Panel (deprecated) objs are valid
  </code></pre>
  
  <p>I just want to add a string value <code>string = 'cool'</code> into the dataframe, but I get this error.</p>