title: TypeError: 'int' object is not iterable, why it's happening
body: <p>Here is three examples actually.</p>
  
  <pre><code>&gt;&gt;&gt; result = []
  &gt;&gt;&gt; for k in range(10):
  &gt;&gt;&gt;    result += k*k
  
  &gt;&gt;&gt; result = []
  &gt;&gt;&gt; for k in range(10):
  &gt;&gt;&gt;    result.append(k*k)
  
  &gt;&gt;&gt; result = [k*k for k in range(10)]
  </code></pre>
  
  <p>First one makes a error. Error prints like below</p>
  
  <pre><code>TypeError: 'int' object is not iterable
  </code></pre>
  
  <p>However, second and third one works well. </p>
  
  <p>I could not understand the difference between those three statements. </p>