title: How is this valid Python syntax?
body: <p>The following is part of SqlAlchemy, I'm trying to figure out how this is valid Python syntax.  I know the code works, I'm just trying to understand what is is doing here and how that is valid.  These are NOT nested classes, they are stand alone classes.  I would have expected a pass after each class declaration though and I'm pretty sure it not being there is significant.</p>
  
  <pre><code>class InstrumentedList(list):
      """An instrumented version of the built-in list."""
  
  
  class InstrumentedSet(set):
      """An instrumented version of the built-in set."""
  
  
  class InstrumentedDict(dict):
      """An instrumented version of the built-in dict."""
  
  
  __canned_instrumentation = {
      list: InstrumentedList,
      set: InstrumentedSet,
      dict: InstrumentedDict,
  }
  
  __interfaces = {
      list: (
          {'appender': 'append', 'remover': 'remove',
           'iterator': '__iter__'}, _list_decorators()
      ),
  
      set: ({'appender': 'add',
             'remover': 'remove',
             'iterator': '__iter__'}, _set_decorators()
            ),
  
      # decorators are required for dicts and object collections.
      dict: ({'iterator': 'values'}, _dict_decorators()) if util.py3k
      else ({'iterator': 'itervalues'}, _dict_decorators()),
  }
  </code></pre>