title: Remove an item from a dictionary when its key is unknown
body: <p>What is the best way to remove an item from a dictionary by value, i.e. when the item's key is unknown?  Here's a simple approach:</p>
  
  <pre><code>for key, item in some_dict.items():
      if item is item_to_remove:
          del some_dict[key]
  </code></pre>
  
  <p>Are there better ways?  Is there anything wrong with mutating (deleting items) from the dictionary while iterating it?</p>