title: Equivalent of setdefault in Go?
body: <p>I can do:</p>
  
  <pre><code>_, ok := some_go_map[a_key]
  </code></pre>
  
  <p>to test for existence of key.</p>
  
  <p>But I've been spoiled by Python's dict's <code>setdefault</code> method (if a key does not have value set in a "map" [dict == associative array], set it to a given default, then get it; otherwise just get).</p>
  
  <p>Wondering if there's some idiom in Go to achieve the same thing?</p>