old_contents: 
  
  class KeepContext(object):
      """
      Context manager that receives a `django.template.Context` instance and a list of keys
  
      Once the context manager is exited, it removes `keys` from the context, to avoid
      side effects in later layout objects that may use the same context variables.
  
      Layout objects should use `extra_context` to introduce context variables, never
      touch context object themselves, that could introduce side effects.
      """
      def __init__(self, context, keys):
          self.context = context
          self.keys = keys
  
      def __enter__(self):
          pass
  
      def __exit__(self, type, value, traceback):
          for key in list(self.keys):
              del self.context[key]
  
new_contents: 
  
  class KeepContext(object):
      """
      Context manager that receives a `django.template.Context` instance and a list of keys
  
      Once the context manager is exited, it removes `keys` from the context, to avoid
      side effects in later layout objects that may use the same context variables.
  
      Layout objects should use `extra_context` to introduce context variables, never
      touch context object themselves, that could introduce side effects.
      """
      def __init__(self, context, keys):
          self.context = context
          self.keys = keys
  
      def __enter__(self):
          pass
  
      def __exit__(self, type, value, traceback):
          for key in list(self.keys):
              if key in self.context:
                  del self.context[key]
  
subject: Fix for #511