code: def event_before_read_docs(app, env, _):
      
      client_id = app.config['imgur_client_id']
      if not client_id:
          raise ExtensionError('imgur_client_id config value must be set for Imgur API calls.')
      if not RE_CLIENT_ID.match(client_id):
          raise ExtensionError('imgur_client_id config value must be 5-30 lower case hexadecimal characters only.')
  
      imgur_album_cache = getattr(env, 'imgur_album_cache', None)
      imgur_image_cache = getattr(env, 'imgur_image_cache', None)
      env.imgur_album_cache, env.imgur_image_cache = initialize(imgur_album_cache, imgur_image_cache, (), ())
      prune_cache(env.imgur_album_cache, env.imgur_image_cache, app)
docs: Called by Sphinx before phase 1 (reading).
  
      * Verify config.
      * Initialize the cache dict before reading any docs with an empty dictionary if not exists.
      * Prune old/invalid data from cache.
  
      :param sphinx.application.Sphinx app: Sphinx application object.
      :param sphinx.environment.BuildEnvironment env: Sphinx build environment.
      :param _: Not used.