title: Remove certain keys from a dictionary in python
body: <p>I'm trying to construct a dictionary that contains a series of sets:</p>
  
  <pre><code>{Field1:{Value1, Value2, Value3}, Field2{Value4}}
  </code></pre>
  
  <p>The trouble is, I then wish to delete any fields from the dictionary that only have one value in the set.  I have been writing code like this:</p>
  
  <pre><code>for field in FieldSet:
      if len(FieldSet[field]) == 1:
          del(FieldSet[field])
  </code></pre>
  
  <p>But receive the error "RuntimeError: dictionary changed size during execution".  (Not surprising, since that's what I'm doing.)  It's not the be-all and end-all if I have to knock together some sort of workaround, but is it possible to do this?</p>