gh-140938: Raise ValueError for infinite inputs to stdev/pstdev (GH-141531)

Raise ValueError for infinite inputs to stdev/pstdev

---

Co-authored-by: Gregory P. Smith <68491+gpshead@users.noreply.github.com>
Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

Lib/statistics.py

@@ -619,9 +619,14 @@ def stdev(data, xbar=None):
     if n < 2:
         raise StatisticsError('stdev requires at least two data points')
     mss = ss / (n - 1)
+    try:
+        mss_numerator = mss.numerator
+        mss_denominator = mss.denominator
+    except AttributeError:
+        raise ValueError('inf or nan encountered in data')
     if issubclass(T, Decimal):
-        return _decimal_sqrt_of_frac(mss.numerator, mss.denominator)
-    return _float_sqrt_of_frac(mss.numerator, mss.denominator)
+        return _decimal_sqrt_of_frac(mss_numerator, mss_denominator)
+    return _float_sqrt_of_frac(mss_numerator, mss_denominator)
 
 
 def pstdev(data, mu=None):
@@ -637,9 +642,14 @@ def pstdev(data, mu=None):
     if n < 1:
         raise StatisticsError('pstdev requires at least one data point')
     mss = ss / n
+    try:
+        mss_numerator = mss.numerator
+        mss_denominator = mss.denominator
+    except AttributeError:
+        raise ValueError('inf or nan encountered in data')
     if issubclass(T, Decimal):
-        return _decimal_sqrt_of_frac(mss.numerator, mss.denominator)
-    return _float_sqrt_of_frac(mss.numerator, mss.denominator)
+        return _decimal_sqrt_of_frac(mss_numerator, mss_denominator)
+    return _float_sqrt_of_frac(mss_numerator, mss_denominator)
 
 
 ## Statistics for relations between two inputs #############################

Lib/test/test_statistics.py

@@ -2005,7 +2005,6 @@ def test_iter_list_same(self):
         expected = self.func(data)
         self.assertEqual(self.func(iter(data)), expected)
 
-
 class TestPVariance(VarianceStdevMixin, NumericTestCase, UnivariateTypeMixin):
     # Tests for population variance.
     def setUp(self):
@@ -2113,6 +2112,14 @@ def test_center_not_at_mean(self):
         self.assertEqual(self.func(data), 2.5)
         self.assertEqual(self.func(data, mu=0.5), 6.5)
 
+    def test_gh_140938(self):
+        # Inputs with inf/nan should raise a ValueError
+        with self.assertRaises(ValueError):
+            self.func([1.0, math.inf])
+        with self.assertRaises(ValueError):
+            self.func([1.0, math.nan])
+
+
 class TestSqrtHelpers(unittest.TestCase):
 
     def test_integer_sqrt_of_frac_rto(self):

Misc/NEWS.d/next/Library/2025-11-13-14-51-30.gh-issue-140938.kXsHHv.rst

@@ -0,0 +1,2 @@
+The :func:`statistics.stdev` and :func:`statistics.pstdev` functions now raise a
+:exc:`ValueError` when the input contains an infinity or a NaN.