code: def _build_provider_list():
      
      registry = None
      if appsettings.FLUENT_OEMBED_SOURCE == 'basic':
          registry = bootstrap_basic()
      elif appsettings.FLUENT_OEMBED_SOURCE == 'embedly':
          params = {}
          if appsettings.MICAWBER_EMBEDLY_KEY:
              params['key'] = appsettings.MICAWBER_EMBEDLY_KEY
          registry = bootstrap_embedly(**params)
      elif appsettings.FLUENT_OEMBED_SOURCE == 'noembed':
          registry = bootstrap_noembed(nowrap=1)
      elif appsettings.FLUENT_OEMBED_SOURCE == 'list':
          # Fill list manually in the settings, e.g. to have a fixed set of supported secure providers.
          registry = ProviderRegistry()
          for regex, provider in appsettings.FLUENT_OEMBED_PROVIDER_LIST:
              registry.register(regex, Provider(provider))
      else:
          raise ImproperlyConfigured("Invalid value of FLUENT_OEMBED_SOURCE, only 'basic', 'list', 'noembed' or 'embedly' is supported.")
  
      # Add any extra providers defined in the settings
      for regex, provider in appsettings.FLUENT_OEMBED_EXTRA_PROVIDERS:
          registry.register(regex, Provider(provider))
  
      return registry
docs: Construct the provider registry, using the app settings.