title: Why chaining augmented assignment throws a SyntaxError
body: <p>Here's the snippet that I'm trying in the Python3 interpreter,</p>
  
  <pre><code>&gt;&gt;&gt; x = y = 3
  &gt;&gt;&gt; x, y
  (3, 3)
  &gt;&gt;&gt; x = y = 3
  &gt;&gt;&gt; x, y
  (3, 3)
  &gt;&gt;&gt; x = y = y + 3
  &gt;&gt;&gt; x, y
  (6, 6)
  &gt;&gt;&gt; x = y +=  3
    File "&lt;stdin&gt;", line 1
      x = y +=  3
             ^
  SyntaxError: invalid syntax
  </code></pre>
  
  <p>I know that SyntaxError arises when the Python grammar doesn't support the expression, but I'm not able to figure out why exactly <code>+=</code> can't be chained like <code>=</code>.</p>