title: Python 'NoneType' itterable in list operation
body: <p>I am writing a function to remove duplication in one list. For specific:</p>
  <blockquote>
  <p>input : a list</p>
  <p>output: a list without any duplication</p>
  </blockquote>
  <p>this is my code:</p>
  <pre><code>def remove_duplicate(s_list):
      for member in s_list:
          for other in s_list.remove(member):
              if other == member:
                  s_list.remove(other)
      return s_list
  </code></pre>
  <p>After that, I try to run <code>remove_duplicate([1,2,3,4])</code> and the error is</p>
  <pre><code>for other in s_list.remove(member):
    TypeError: 'NoneType' object is not iterable
  </code></pre>