old_contents: #!/usr/bin/env python
  # -*- coding: utf-8 -*-
  
  """
  Medias locator
  ==============
  """
  
  from __future__ import absolute_import
  
  import os.path
  
  
  class PathMediasLocator(object):
      """
      Returns medias relatively to the root directory *base*.
      """
      def __init__(self, base):
          self.base = base
  
      def __call__(self, path):
          path = path.lstrip('/')
          return os.path.join(self.base, path)
  
  
  class NoMediasLocator(object):
      """
      Raises an error when a media is asked.
      """
      def __call__(self, path):
          raise RuntimeError('No media path')
  
new_contents: #!/usr/bin/env python
  # -*- coding: utf-8 -*-
  
  """
  Medias locator
  ==============
  
  Medias locator returns a path on the file system from the *src* of an img tag.
  
  .. data:: PLACEHOLDER
  
      A special object that indicates to the renderer to use a placeholder
      instead of a media.
  """
  
  from __future__ import absolute_import
  
  import os.path
  
  
  PLACEHOLDER = object()
  
  
  class PathMediasLocator(object):
      """
      Returns medias relatively to the root directory *base*.
      """
      def __init__(self, base):
          self.base = base
  
      def __call__(self, path):
          path = path.lstrip('/')
          return os.path.join(self.base, path)
  
  
  class NoMediasLocator(object):
      """
      Raises an error when a media is asked.
      """
      def __call__(self, path):
          raise RuntimeError('No media path')
  
  
  class DebugMediasLocator(object):
      """
      Return :data:`PLACEHOLDER`
      """
  
      def __call__(self, path):
          return PLACEHOLDER
  
subject: Add a special PLACEHOLDER value