bpo-36018: Add properties for mean and stdev (GH-12022)

Responding to suggestions on the tracker and some off-line suggestions.

Davin suggested that english named accessors instead of greek letters would result in more intelligible user code. Steven suggested that the parameters still need to be *mu* and *theta* which are used elsewhere (and I noted those parameter names are used in linked-to resources).

Michael suggested proving-out the API by seeing whether it generalized to *Lognormal*.  I did so and found that Lognormal distribution parameters *mu* and *sigma*  do not represent the mean and standard deviation of the lognormal distribution (instead, they are for the underlying regular normal distribution).

Putting these ideas together, we have NormalDist parameterized by *mu* and *sigma* but offering English named properties for accessors.  That gives lets us match other API that access mu and sigma, it matches the external resources on the topic, gives us clear english names in user code. The API extends nicely to LogNormal where the parameters and the summary statistic accessors are not the same.

https://bugs.python.org/issue36018

diff --git a/Doc/library/statistics.rst b/Doc/library/statistics.rst
index c1be295cbf95022c4cbcf7ca17dfdaeb3caee27a..a0d4d3910220088052a80eac16b0b25599cb4268 100644 (file)
--- a/Doc/library/statistics.rst
+++ b/Doc/library/statistics.rst
@@ -488,13 +488,17 @@ of applications in statistics, including simulations and hypothesis testing.
 
     If *sigma* is negative, raises :exc:`StatisticsError`.
 
-    .. attribute:: mu
+    .. attribute:: mean
 
-        The mean of a normal distribution.
+       A read-only property representing the `arithmetic mean
+       <https://en.wikipedia.org/wiki/Arithmetic_mean>`_ of a normal
+       distribution.
 
-    .. attribute:: sigma
+    .. attribute:: stdev
 
-        The standard deviation of a normal distribution.
+       A read-only property representing the `standard deviation
+       <https://en.wikipedia.org/wiki/Standard_deviation>`_ of a normal
+       distribution.
 
     .. attribute:: variance
 
@@ -566,8 +570,8 @@ of applications in statistics, including simulations and hypothesis testing.
         >>> birth_weights = NormalDist.from_samples([2.5, 3.1, 2.1, 2.4, 2.7, 3.5])
         >>> drug_effects = NormalDist(0.4, 0.15)
         >>> combined = birth_weights + drug_effects
-        >>> f'mu={combined.mu :.1f}   sigma={combined.sigma :.1f}'
-        'mu=3.1   sigma=0.5'
+        >>> f'mean: {combined.mean :.1f}   standard deviation: {combined.stdev :.1f}'
+        'mean: 3.1   standard deviation: 0.5'
 
     .. versionadded:: 3.8
 

diff --git a/Lib/statistics.py b/Lib/statistics.py
index bf10e19c0d4138d997d13d3d86eec34233f6a7a1..bab585750d9124ce91785183c9d5feb20e542e7f 100644 (file)
--- a/Lib/statistics.py
+++ b/Lib/statistics.py
@@ -740,6 +740,16 @@ class NormalDist:
             raise StatisticsError('cdf() not defined when sigma is zero')
         return 0.5 * (1.0 + erf((x - self.mu) / (self.sigma * sqrt(2.0))))
 
+    @property
+    def mean(self):
+        'Arithmetic mean of the normal distribution'
+        return self.mu
+
+    @property
+    def stdev(self):
+        'Standard deviation of the normal distribution'
+        return self.sigma
+
     @property
     def variance(self):
         'Square of the standard deviation'

diff --git a/Lib/test/test_statistics.py b/Lib/test/test_statistics.py
index 9549240f9092462f7d475f9c46b4ddb996b00baf..d35cdd8420a37638ee40f949c35c40e77a75170c 100644 (file)
--- a/Lib/test/test_statistics.py
+++ b/Lib/test/test_statistics.py
@@ -2128,6 +2128,12 @@ class TestNormalDist(unittest.TestCase):
         with self.assertRaises(statistics.StatisticsError):
             Y.cdf(90)
 
+    def test_properties(self):
+        X = statistics.NormalDist(100, 15)
+        self.assertEqual(X.mean, 100)
+        self.assertEqual(X.stdev, 15)
+        self.assertEqual(X.variance, 225)
+
     def test_unary_operations(self):
         NormalDist = statistics.NormalDist
         X = NormalDist(100, 12)