title: Convert Nested list to str
body: <p>I want to conver a nested list to str. i am using for loop and ','.join method. but how t can be acheived in a effective way</p>
  <pre><code>input= [[[[['a', 'b'], 'c'], 'd'], 'e'], 'f']
  output = a,b,c,d,e,f 
  </code></pre>