lenskit.metrics.ListMetric
==========================

.. py:class:: lenskit.metrics.ListMetric
   :canonical: lenskit.metrics._base.ListMetric

   Bases: :py:obj:`Metric`\ [\ :py:obj:`float | None`\ , :py:obj:`lenskit.data.accum.ValueStatistics`\ ]


   Base class for metrics defined on individual recommendation outputs.  This
   is the most common type of metric.

   For prediction metrics, this is *macro-averaging*.

   Metrics based on this class implement :meth:`measure_list` to compute a
   single numeric value for each list, and the accumulated result will be basic
   statistical summaries of those values.

   :Stability: Full


   .. py:attribute:: default
      :type:  ClassVar[float | None]
      :value: 0.0



   .. py:method:: measure_list(output, test, /)
      :abstractmethod:


      Compute measurements for a single list.

      :returns:

                - A float for simple metrics
                - Intermediate data for decomposed metrics
                - A dict mapping metric names to values for multi-metric classes



   .. py:method:: extract_list_metrics(data, /)

      Return the given per-list metric result.



   .. py:method:: create_accumulator()

      Creaet an accumulator to aggregate per-list measurements into summary
      metrics.

      Each result from :meth:`measure_list` is passed to
      :meth:`Accumulator.add`.