title: python pandas: find the element that is a string in a pandas DataFrame
body: <p>Say i have a pandas DataFrame: </p>
  
  <pre><code>df = pd.DataFrame({'a': [1,2,3,'e',4], 'b': [1,2,3,4,5]})
  </code></pre>
  
  <p>I would like to have the index of where the element of df is a string. 
  How can i do that other than by checking element after element
  which slow and inefficient?</p>