bpo-36018: Address more reviewer feedback

Doc/library/statistics.rst

@@ -514,15 +514,14 @@ However, for reading convenience, most of the examples show sorted sequences.
 
    Set *n* to 4 for quartiles (the default).  Set *n* to 10 for deciles.  Set
    *n* to 100 for percentiles which gives the 99 cuts points that separate
-   *data* in to 100 equal sized groups.  Raises :exc:`StatisticsError` if *n*
+   *data* into 100 equal sized groups.  Raises :exc:`StatisticsError` if *n*
    is not least 1.
 
-   The *data* can be any iterable containing sample data or it can be an
-   instance of a class that defines an :meth:`~inv_cdf` method.  For meaningful
+   The *data* can be any iterable containing sample data.  For meaningful
    results, the number of data points in *data* should be larger than *n*.
    Raises :exc:`StatisticsError` if there are not at least two data points.
 
-   For sample data, the cut points are linearly interpolated from the
+   The cut points are linearly interpolated from the
    two nearest data points.  For example, if a cut point falls one-third
    of the distance between two sample values, ``100`` and ``112``, the
    cut-point will evaluate to ``104``.
@@ -547,9 +546,6 @@ However, for reading convenience, most of the examples show sorted sequences.
    values, the method sorts them and assigns the following percentiles:
    0%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 100%.
 
-   If *data* is an instance of a class that defines an
-   :meth:`~inv_cdf` method, setting *method* has no effect.
-
    .. doctest::
 
         # Decile cut points for empirically sampled data
@@ -561,11 +557,6 @@ However, for reading convenience, most of the examples show sorted sequences.
         >>> [round(q, 1) for q in quantiles(data, n=10)]
         [81.0, 86.2, 89.0, 99.4, 102.5, 103.6, 106.0, 109.8, 111.0]
 
-        >>> # Quartile cut points for the standard normal distribution
-        >>> Z = NormalDist()
-        >>> [round(q, 4) for q in quantiles(Z, n=4)]
-        [-0.6745, 0.0, 0.6745]
-
    .. versionadded:: 3.8
 
 
@@ -607,6 +598,18 @@ of applications in statistics.
        <https://en.wikipedia.org/wiki/Arithmetic_mean>`_ of a normal
        distribution.
 
+    .. attribute:: median
+
+       A read-only property for the `median
+       <https://en.wikipedia.org/wiki/Median>`_ of a normal
+       distribution.
+
+    .. attribute:: mode
+
+       A read-only property for the `mode
+       <https://en.wikipedia.org/wiki/Mode_(statistics)>`_ of a normal
+       distribution.
+
     .. attribute:: stdev
 
        A read-only property for the `standard deviation
@@ -678,6 +681,16 @@ of applications in statistics.
        the two probability density functions
        <https://www.rasch.org/rmt/rmt101r.htm>`_.
 
+    .. method:: NormalDist.quantiles()
+
+        Divide the normal distribution into *n* continuous intervals with
+        equal probability.  Returns a list of (n - 1) cut points separating
+        the intervals.
+
+        Set *n* to 4 for quartiles (the default).  Set *n* to 10 for deciles.
+        Set *n* to 100 for percentiles which gives the 99 cuts points that
+        separate the normal distribution into 100 equal sized groups.
+
     Instances of :class:`NormalDist` support addition, subtraction,
     multiplication and division by a constant.  These operations
     are used for translation and scaling.  For example:
@@ -733,9 +746,9 @@ Find the `quartiles <https://en.wikipedia.org/wiki/Quartile>`_ and `deciles
 
 .. doctest::
 
-    >>> list(map(round, quantiles(sat)))
+    >>> list(map(round, sat.quantiles()))
     [928, 1060, 1192]
-    >>> list(map(round, quantiles(sat, n=10)))
+    >>> list(map(round, sat.quantiles(n=10)))
     [810, 896, 958, 1011, 1060, 1109, 1162, 1224, 1310]
 
 To estimate the distribution for a model than isn't easy to solve

Lib/statistics.py

@@ -624,18 +624,15 @@ def quantiles(data, /, *, n=4, method='exclusive'):
     Set *n* to 100 for percentiles which gives the 99 cuts points that
     separate *data* in to 100 equal sized groups.
 
-    The *data* can be any iterable containing sample data or it can be
-    an instance of a class that defines an inv_cdf() method.  For sample
-    data, the cut points are linearly interpolated between data points.
+    The *data* can be any iterable containing sample.
+    The cut points are linearly interpolated between data points.
 
     If *method* is set to *inclusive*, *data* is treated as population
     data.  The minimum value is treated as the 0th percentile and the
     maximum value is treated as the 100th percentile.
     """
     if n < 1:
         raise StatisticsError('n must be at least 1')
-    if hasattr(data, 'inv_cdf'):
-        return [data.inv_cdf(i / n) for i in range(1, n)]
     data = sorted(data)
     ld = len(data)
     if ld < 2:
@@ -955,6 +952,17 @@ def inv_cdf(self, p):
             raise StatisticsError('cdf() not defined when sigma at or below zero')
         return _normal_dist_inv_cdf(p, self._mu, self._sigma)
 
+    def quantiles(self, n=4):
+        """Divide into *n* continuous intervals with equal probability.
+
+        Returns a list of (n - 1) cut points separating the intervals.
+
+        Set *n* to 4 for quartiles (the default).  Set *n* to 10 for deciles.
+        Set *n* to 100 for percentiles which gives the 99 cuts points that
+        separate the normal distribution in to 100 equal sized groups.
+        """
+        return [self.inv_cdf(i / n) for i in range(1, n)]
+
     def overlap(self, other):
         """Compute the overlapping coefficient (OVL) between two normal distributions.
 
@@ -994,6 +1002,20 @@ def mean(self):
         "Arithmetic mean of the normal distribution."
         return self._mu
 
+    @property
+    def median(self):
+        "Return the median of the normal distribution"
+        return self._mu
+
+    @property
+    def mode(self):
+        """Return the mode of the normal distribution
+
+        The mode is the value x where which the probability density
+        function (pdf) takes its maximum value.
+        """
+        return self._mu
+
     @property
     def stdev(self):
         "Standard deviation of the normal distribution."

Lib/test/test_statistics.py

@@ -2198,16 +2198,6 @@ def f(x):
             exp = list(map(f, expected))
             act = quantiles(map(f, data), n=n)
             self.assertTrue(all(math.isclose(e, a) for e, a in zip(exp, act)))
-        # Quartiles of a standard normal distribution
-        for n, expected in [
-            (1, []),
-            (2, [0.0]),
-            (3, [-0.4307, 0.4307]),
-            (4 ,[-0.6745, 0.0, 0.6745]),
-                ]:
-            actual = quantiles(statistics.NormalDist(), n=n)
-            self.assertTrue(all(math.isclose(e, a, abs_tol=0.0001)
-                            for e, a in zip(expected, actual)))
         # Q2 agrees with median()
         for k in range(2, 60):
             data = random.choices(range(100), k=k)
@@ -2248,16 +2238,6 @@ def f(x):
             exp = list(map(f, expected))
             act = quantiles(map(f, data), n=n, method="inclusive")
             self.assertTrue(all(math.isclose(e, a) for e, a in zip(exp, act)))
-        # Quartiles of a standard normal distribution
-        for n, expected in [
-            (1, []),
-            (2, [0.0]),
-            (3, [-0.4307, 0.4307]),
-            (4 ,[-0.6745, 0.0, 0.6745]),
-                ]:
-            actual = quantiles(statistics.NormalDist(), n=n, method="inclusive")
-            self.assertTrue(all(math.isclose(e, a, abs_tol=0.0001)
-                            for e, a in zip(expected, actual)))
         # Natural deciles
         self.assertEqual(quantiles([0, 100], n=10, method='inclusive'),
                          [10.0, 20.0, 30.0, 40.0, 50.0, 60.0, 70.0, 80.0, 90.0])
@@ -2546,6 +2526,19 @@ def test_inv_cdf(self):
         # Special values
         self.assertTrue(math.isnan(Z.inv_cdf(float('NaN'))))
 
+    def test_quantiles(self):
+        # Quartiles of a standard normal distribution
+        Z = self.module.NormalDist()
+        for n, expected in [
+            (1, []),
+            (2, [0.0]),
+            (3, [-0.4307, 0.4307]),
+            (4 ,[-0.6745, 0.0, 0.6745]),
+                ]:
+            actual = Z.quantiles(n=n)
+            self.assertTrue(all(math.isclose(e, a, abs_tol=0.0001)
+                            for e, a in zip(expected, actual)))
+
     def test_overlap(self):
         NormalDist = self.module.NormalDist
 
@@ -2612,6 +2605,8 @@ def overlap_numeric(X, Y, *, steps=8_192, z=5):
     def test_properties(self):
         X = self.module.NormalDist(100, 15)
         self.assertEqual(X.mean, 100)
+        self.assertEqual(X.median, 100)
+        self.assertEqual(X.mode, 100)
         self.assertEqual(X.stdev, 15)
         self.assertEqual(X.variance, 225)