code: def mix(self, cls):
          
  
          if hasattr(cls, 'unmixed_class'):
              base_class = cls.unmixed_class
              old_mixins = cls.__bases__[1:]  # Skip the original unmixed class
              mixins = old_mixins + tuple(
                  mixin
                  for mixin in self._mixins
                  if mixin not in old_mixins
              )
          else:
              base_class = cls
              mixins = self._mixins
  
          mixin_key = (base_class, mixins)
  
          if mixin_key not in _CLASS_CACHE:
              # Only lock if we're about to make a new class
              with _CLASS_CACHE_LOCK:
                  # Use setdefault so that if someone else has already
                  # created a class before we got the lock, we don't
                  # overwrite it
                  return _CLASS_CACHE.setdefault(mixin_key, type(
                      base_class.__name__ + str('WithMixins'),  # type() requires native str
                      (base_class, ) + mixins,
                      {'unmixed_class': base_class}
                  ))
          else:
              return _CLASS_CACHE[mixin_key]
docs: Returns a subclass of `cls` mixed with `self.mixins`.
  
          :param cls: The base class to mix into
          :type cls: `class`