title: Erroneous behaviour while updating nested dictionary python3
body: <p>While working on defaultdict class of collection package in python3.7, I see that new key is generated from the duplicate of last key, instead of initiating dictionary. Is there a way to initiate new element with given dictionary which is <code>init_dict</code> in below example code.</p>
  <p>Example code to reproduce error:</p>
  <pre><code>from collections import defaultdict
  init_dict = {'buy_qty': 0, 
               'sell_qty': 0}
  
  pnl = defaultdict(lambda: init_dict)
  pnl['a']['buy_qty'] += 1
  pnl['a']['sell_qty'] += 1
  </code></pre>
  <p>Now when I do</p>
  <pre><code>pnl['b'] 
  </code></pre>
  <p>gives me</p>
  <pre><code>{'buy_qty': 1, 'sell_qty': 1}
  </code></pre>
  <p>I am looking for <code>pnl['b']</code> to be initialized with <code>init_dict</code>. How can I achieve that?</p>