Add comment and beef-up tests

python · rhettinger · Nov 27, 2021 · Mar 16, 2021 · Mar 22, 2021 · Mar 22, 2021
commit 28fdccf215c7e65fa4cb3038d0b634bcbbb29134
diff --git a/Lib/statistics.py b/Lib/statistics.py
@@ -311,6 +311,7 @@ def _isqrt_frac_rto(n: int, m: int) -> float:
 
 def _sqrt_frac(n: int, m: int) -> float:
     'Square root of n/m as a float, correctly rounded.'
+    # The constant 109 is:  3 + 2 * sys.float_info.mant_dig
     q: int = (n.bit_length() - m.bit_length() - 109) // 2
     if q >= 0:
         return float(_isqrt_frac_rto(n, m << 2 * q) << q)

diff --git a/Lib/test/test_statistics.py b/Lib/test/test_statistics.py
@@ -15,7 +15,7 @@
 import sys
 import unittest
 from test import support
-from test.support import import_helper
+from test.support import import_helper, requires_IEEE_754
 
 from decimal import Decimal
 from fractions import Fraction
@@ -2163,6 +2163,7 @@ def test_center_not_at_mean(self):
 
 class TestSqrtHelper(unittest.TestCase):
 
+    @requires_IEEE_754
     def test_sqrt_frac(self):
 
         def is_root_correctly_rounded(x: Fraction, root: float) -> bool:
@@ -2189,6 +2190,16 @@ def is_root_correctly_rounded(x: Fraction, root: float) -> bool:
                 root: float = statistics._sqrt_frac(numerator, denonimator)
                 self.assertTrue(is_root_correctly_rounded(x, root))
 
+        # Verify that corner cases and error handling match math.sqrt()
+        self.assertEqual(statistics._sqrt_frac(0, 1), 0.0)
+        with self.assertRaises(ValueError):
+            statistics._sqrt_frac(-1, 1)
+        with self.assertRaises(ValueError):
+            statistics._sqrt_frac(1, -1)
+
+        # The result is well defined if both inputs are negative
+        self.assertAlmostEqual(statistics._sqrt_frac(-2, -1), math.sqrt(2.0))
+
 
 class TestStdev(VarianceStdevMixin, NumericTestCase):
     # Tests for sample standard deviation.