title: Adding multidict values to a list
body: <p>I am sending data from an HTML form, and I handle it with python(Pyramid framework), this is what I have in my view:</p>
  
  <pre><code> @view_config(renderer='json', request_method='POST')
      def modify(self):
          d = self.request.params
          if d.get("perms"):
              if type(d.get("perms")) == str or type(d.get("perms")) == unicode:
                  d["perms"] = [d["perms"]]
              for perm in d["perms"]:
                  d[perm] = "on"
  </code></pre>
  
  <p>When I try to do <code>d["perms"] = [d["perms"]]</code>, I get an error:</p>
  
  <pre><code>KeyError: 'NestedMultiDict objects are read-only'
  </code></pre>
  
  <p>I have tried to change the above piece of code to:</p>
  
  <pre><code>perms = []
  for k, v in d.iteritems():
      if k == "perms":
          if type(v) == str or type(v) == unicode:
              perms = [v]
          for perm in perms:
              d[perm] = "on"
  </code></pre>
  
  <p>But it gives me the same error.</p>
  
  <p>Is it possible to add a <code>MultiDict</code> value to a list? 
  If so, how?
  Why is a <code>MultiDict</code> read-only?</p>