title: Index of substring in a python list of strings
body: <p>How can I extract the index of a substring in a <code>python</code> list of strings (preferentially in a rapid way to handle long lists)?</p>
  
  <p>For example, with <code>mylist = ['abc', 'day', 'ghi']</code> and character <code>'a'</code>, I would like to return <code>[0, 1, -1]</code>.</p>