title: Why is extended slice assignment less flexible than regular slice assignment?
body: <p>According to the Python documentation on <a href="https://docs.python.org/3/whatsnew/2.3.html#extended-slices" rel="noreferrer">extended slices</a>:</p>
  
  <blockquote>
    <p>If you have a mutable sequence such as a list or an array you can
    assign to or delete an extended slice, but there are some differences
    between assignment to extended and regular slices. Assignment to a
    regular slice can be used to change the length of the sequence:</p>
  
  <pre><code>&gt;&gt;&gt; a = range(3)
  &gt;&gt;&gt; a
  [0, 1, 2]
  &gt;&gt;&gt; a[1:3] = [4, 5, 6]
  &gt;&gt;&gt; a
  [0, 4, 5, 6]
  </code></pre>
    
    <p>Extended slices aren't this flexible. When assigning to an extended
    slice, the list on the right hand side of the statement must contain
    the same number of items as the slice it is replacing:</p>
  
  <pre><code>&gt;&gt;&gt; a = range(4)
  &gt;&gt;&gt; a
  [0, 1, 2, 3]
  &gt;&gt;&gt; a[::2]
  [0, 2]
  &gt;&gt;&gt; a[::2] = [0, -1]
  &gt;&gt;&gt; a
  [0, 1, -1, 3]
  &gt;&gt;&gt; a[::2] = [0,1,2]
  Traceback (most recent call last):
    File "&lt;stdin&gt;", line 1, in ?
  ValueError: attempt to assign sequence of size 3 to extended slice of size 2
  </code></pre>
  </blockquote>
  
  <p>I do not understand why the "ordinary" slice method works but the "extended" slice method doesn't work. What differentiates an "ordinary" slice from an "extended" slice, and why does the "extended" slice method fail?</p>