title: Split elements of a list inside a list in python
body: <p>I am new to Python. How can I make a single list out of many lists inside a list?<br>
  For example,</p>
  
  <pre><code>list1 = ['aplle', 'grape', ['aplle1', 'grape1'] ]
  </code></pre>
  
  <p>Output should be:</p>
  
  <pre><code>list1 = ['aplle', 'grape', 'aplle1', 'grape1']
  </code></pre>
  
  <p>The list has thousands of elements.</p>