title: Why is the attribute of dict immutable object, and the list in it is mutable object
body: <p>I was watching explore python. It had an example:</p>
  
  <pre><code>    &gt;&gt;&gt; d1 = {'name': 'ethan', 'books': ['book1', 'book2', 'book3']}
      &gt;&gt;&gt; d2 = d1.copy()
      &gt;&gt;&gt; d2['name'] = 'peter'         
      &gt;&gt;&gt; d2
      {'books': ['book1', 'book2', 'book3'], 'name': 'peter'}
      &gt;&gt;&gt; d1
      {'books': ['book1', 'book2', 'book3'], 'name': 'Ethan'}
  </code></pre>
  
  <p>Is a dict not a value-key object? Why it said the value of name is an immutable object, so the properties of d1 will not change.</p>