bpo-35892: Fix mode() and add multimode() (#12089)

2 files changed, 38 insertions, 8 deletions

diff --git a/Doc/library/statistics.rst b/Doc/library/statistics.rst
index 81119da0a3..97e1c3a0a1 100644
--- a/Doc/library/statistics.rst
+++ b/Doc/library/statistics.rst
@@ -37,7 +37,7 @@ Averages and measures of central location
 These functions calculate an average or typical value from a population
 or sample.
 
-=======================  =============================================
+=======================  ===============================================================
 :func:`mean`             Arithmetic mean ("average") of data.
 :func:`fmean`            Fast, floating point arithmetic mean.
 :func:`harmonic_mean`    Harmonic mean of data.
@@ -45,8 +45,9 @@ or sample.
 :func:`median_low`       Low median of data.
 :func:`median_high`      High median of data.
 :func:`median_grouped`   Median, or 50th percentile, of grouped data.
-:func:`mode`             Mode (most common value) of discrete data.
-=======================  =============================================
+:func:`mode`             Single mode (most common value) of discrete or nominal data.
+:func:`multimode`        List of modes (most common values) of discrete or nomimal data.
+=======================  ===============================================================
 
 Measures of spread
 ------------------
@@ -287,12 +288,12 @@ However, for reading convenience, most of the examples show sorted sequences.
 
 .. function:: mode(data)
 
-   Return the most common data point from discrete or nominal *data*.  The mode
-   (when it exists) is the most typical value, and is a robust measure of
-   central location.
+   Return the single most common data point from discrete or nominal *data*.
+   The mode (when it exists) is the most typical value and serves as a
+   measure of central location.
 
-   If *data* is empty, or if there is not exactly one most common value,
-   :exc:`StatisticsError` is raised.
+   If there are multiple modes, returns the first one encountered in the *data*.
+   If *data* is empty, :exc:`StatisticsError` is raised.
 
    ``mode`` assumes discrete data, and returns a single value. This is the
    standard treatment of the mode as commonly taught in schools:
@@ -310,6 +311,27 @@ However, for reading convenience, most of the examples show sorted sequences.
       >>> mode(["red", "blue", "blue", "red", "green", "red", "red"])
       'red'
 
+   .. versionchanged:: 3.8
+      Now handles multimodal datasets by returning the first mode encountered.
+      Formerly, it raised :exc:`StatisticsError` when more than one mode was
+      found.
+
+
+.. function:: multimode(data)
+
+   Return a list of the most frequently occurring values in the order they
+   were first encountered in the *data*.  Will return more than one result if
+   there are multiple modes or an empty list if the *data* is empty:
+
+   .. doctest::
+
+        >>> multimode('aabbbbccddddeeffffgg')
+        ['b', 'd', 'f']
+        >>> multimode('')
+        []
+
+   .. versionadded:: 3.8
+
 
 .. function:: pstdev(data, mu=None)
 
diff --git a/Doc/whatsnew/3.8.rst b/Doc/whatsnew/3.8.rst
index 9cd5a3a937..ad86917d0c 100644
--- a/Doc/whatsnew/3.8.rst
+++ b/Doc/whatsnew/3.8.rst
@@ -282,6 +282,9 @@ Added :func:`statistics.fmean` as a faster, floating point variant of
 :func:`statistics.mean()`.  (Contributed by Raymond Hettinger and
 Steven D'Aprano in :issue:`35904`.)
 
+Added :func:`statistics.multimode` that returns a list of the most
+common values. (Contributed by Raymond Hettinger in :issue:`35892`.)
+
 Added :class:`statistics.NormalDist`, a tool for creating
 and manipulating normal distributions of a random variable.
 (Contributed by Raymond Hettinger in :issue:`36018`.)
@@ -591,6 +594,11 @@ Changes in the Python API
 * The function :func:`platform.popen` has been removed, it was deprecated since
   Python 3.3: use :func:`os.popen` instead.
 
+* The :func:`statistics.mode` function no longer raises an exception
+  when given multimodal data.  Instead, it returns the first mode
+  encountered in the input data.  (Contributed by Raymond Hettinger
+  in :issue:`35892`.)
+
 * The :meth:`~tkinter.ttk.Treeview.selection` method of the
   :class:`tkinter.ttk.Treeview` class no longer takes arguments.  Using it with
   arguments for changing the selection was deprecated in Python 3.6.  Use