title: Appending a list that receives a list containing letters and returns a new list with all the positions of a given letter
body: <p>So I needed to define a function that receives a list containing letters and returns a new list with all the positions of a given letter, e.g. <code>lst = ['a', 'b', 'c']</code> if <code>letter = 'a'</code> then <code>[0]</code>. In my code I get     </p>
  
  <pre><code>    res=res.append(finallist)
  AttributeError: 'NoneType' object has no attribute 'append'
  </code></pre>
  
  <p>What did I do wrong?</p>
  
  <pre><code>def  searchlist(lst,letter):
     res=[]
     for i in range(len(lst)):
       finallist=lst.index(letter)
       res=res.append(finallist)
     return res
  </code></pre>