Issue 26002 and 25974

patches by Upendra Kumar and Stefan Krah
speed up median by using bisect, and general speedup for Decimals using as_integer_ratio

Lib/statistics.py

@@ -105,6 +105,7 @@ import math
 from fractions import Fraction
 from decimal import Decimal
 from itertools import groupby
+from bisect import bisect_left, bisect_right
 
 
 
@@ -223,56 +224,26 @@ def _exact_ratio(x):
         # Optimise the common case of floats. We expect that the most often
         # used numeric type will be builtin floats, so try to make this as
         # fast as possible.
-        if type(x) is float:
+        if type(x) is float or type(x) is Decimal:
             return x.as_integer_ratio()
         try:
             # x may be an int, Fraction, or Integral ABC.
             return (x.numerator, x.denominator)
         except AttributeError:
             try:
-                # x may be a float subclass.
+                # x may be a float or Decimal subclass.
                 return x.as_integer_ratio()
             except AttributeError:
-                try:
-                    # x may be a Decimal.
-                    return _decimal_to_ratio(x)
-                except AttributeError:
-                    # Just give up?
-                    pass
+                # Just give up?
+                pass
     except (OverflowError, ValueError):
         # float NAN or INF.
-        assert not math.isfinite(x)
+        assert not _isfinite(x)
         return (x, None)
     msg = "can't convert type '{}' to numerator/denominator"
     raise TypeError(msg.format(type(x).__name__))
 
 
-# FIXME This is faster than Fraction.from_decimal, but still too slow.
-def _decimal_to_ratio(d):
-    """Convert Decimal d to exact integer ratio (numerator, denominator).
-
-    >>> from decimal import Decimal
-    >>> _decimal_to_ratio(Decimal("2.6"))
-    (26, 10)
-
-    """
-    sign, digits, exp = d.as_tuple()
-    if exp in ('F', 'n', 'N'):  # INF, NAN, sNAN
-        assert not d.is_finite()
-        return (d, None)
-    num = 0
-    for digit in digits:
-        num = num*10 + digit
-    if exp < 0:
-        den = 10**-exp
-    else:
-        num *= 10**exp
-        den = 1
-    if sign:
-        num = -num
-    return (num, den)
-
-
 def _convert(value, T):
     """Convert value to given numeric type T."""
     if type(value) is T:
@@ -305,6 +276,21 @@ def _counts(data):
     return table
 
 
+def _find_lteq(a, x):
+    'Locate the leftmost value exactly equal to x'
+    i = bisect_left(a, x)
+    if i != len(a) and a[i] == x:
+        return i
+    raise ValueError
+
+
+def _find_rteq(a, l, x):
+    'Locate the rightmost value exactly equal to x'
+    i = bisect_right(a, x, lo=l)
+    if i != (len(a)+1) and a[i-1] == x:
+        return i-1
+    raise ValueError
+
 # === Measures of central tendency (averages) ===
 
 def mean(data):
@@ -442,9 +428,15 @@ def median_grouped(data, interval=1):
     except TypeError:
         # Mixed type. For now we just coerce to float.
         L = float(x) - float(interval)/2
-    cf = data.index(x)  # Number of values below the median interval.
-    # FIXME The following line could be more efficient for big lists.
-    f = data.count(x)  # Number of data points in the median interval.
+
+    # Uses bisection search to search for x in data with log(n) time complexity
+    # Find the position of leftmost occurence of x in data
+    l1 = _find_lteq(data, x)
+    # Find the position of rightmost occurence of x in data[l1...len(data)]
+    # Assuming always l1 <= l2
+    l2 = _find_rteq(data, l1, x)
+    cf = l1
+    f = l2 - l1 + 1
     return L + interval*(n/2 - cf)/f
 
 

Lib/test/test_statistics.py

@@ -699,13 +699,12 @@ class ExactRatioTest(unittest.TestCase):
             num, den = statistics._exact_ratio(x)
             self.assertEqual(x, num/den)
 
-    @unittest.skipIf(True, "temporarily disabled: see #25928")
     def test_decimal(self):
         D = Decimal
         _exact_ratio = statistics._exact_ratio
-        self.assertEqual(_exact_ratio(D("0.125")), (125, 1000))
-        self.assertEqual(_exact_ratio(D("12.345")), (12345, 1000))
-        self.assertEqual(_exact_ratio(D("-1.98")), (-198, 100))
+        self.assertEqual(_exact_ratio(D("0.125")), (1, 8))
+        self.assertEqual(_exact_ratio(D("12.345")), (2469, 200))
+        self.assertEqual(_exact_ratio(D("-1.98")), (-99, 50))
 
     def test_inf(self):
         INF = float("INF")
@@ -731,7 +730,6 @@ class ExactRatioTest(unittest.TestCase):
             self.assertIs(ratio[1], None)
             self.assertEqual(type(ratio[0]), type(nan))
 
-    @unittest.skipIf(True, "temporarily disabled: see #25928")
     def test_decimal_nan(self):
         NAN = Decimal("NAN")
         sNAN = Decimal("sNAN")
@@ -745,18 +743,18 @@ class ExactRatioTest(unittest.TestCase):
 
 
 class DecimalToRatioTest(unittest.TestCase):
-    # Test _decimal_to_ratio private function.
+    # Test _exact_ratio private function.
 
     def test_infinity(self):
         # Test that INFs are handled correctly.
         inf = Decimal('INF')
-        self.assertEqual(statistics._decimal_to_ratio(inf), (inf, None))
-        self.assertEqual(statistics._decimal_to_ratio(-inf), (-inf, None))
+        self.assertEqual(statistics._exact_ratio(inf), (inf, None))
+        self.assertEqual(statistics._exact_ratio(-inf), (-inf, None))
 
     def test_nan(self):
         # Test that NANs are handled correctly.
         for nan in (Decimal('NAN'), Decimal('sNAN')):
-            num, den = statistics._decimal_to_ratio(nan)
+            num, den = statistics._exact_ratio(nan)
             # Because NANs always compare non-equal, we cannot use assertEqual.
             # Nor can we use an identity test, as we don't guarantee anything
             # about the object identity.
@@ -769,30 +767,30 @@ class DecimalToRatioTest(unittest.TestCase):
         for d in numbers:
             # First test positive decimals.
             assert d > 0
-            num, den = statistics._decimal_to_ratio(d)
+            num, den = statistics._exact_ratio(d)
             self.assertGreaterEqual(num, 0)
             self.assertGreater(den, 0)
             # Then test negative decimals.
-            num, den = statistics._decimal_to_ratio(-d)
+            num, den = statistics._exact_ratio(-d)
             self.assertLessEqual(num, 0)
             self.assertGreater(den, 0)
 
     def test_negative_exponent(self):
         # Test result when the exponent is negative.
-        t = statistics._decimal_to_ratio(Decimal("0.1234"))
-        self.assertEqual(t, (1234, 10000))
+        t = statistics._exact_ratio(Decimal("0.1234"))
+        self.assertEqual(t, (617, 5000))
 
     def test_positive_exponent(self):
         # Test results when the exponent is positive.
-        t = statistics._decimal_to_ratio(Decimal("1.234e7"))
+        t = statistics._exact_ratio(Decimal("1.234e7"))
         self.assertEqual(t, (12340000, 1))
 
     def test_regression_20536(self):
         # Regression test for issue 20536.
         # See http://bugs.python.org/issue20536
-        t = statistics._decimal_to_ratio(Decimal("1e2"))
+        t = statistics._exact_ratio(Decimal("1e2"))
         self.assertEqual(t, (100, 1))
-        t = statistics._decimal_to_ratio(Decimal("1.47e5"))
+        t = statistics._exact_ratio(Decimal("1.47e5"))
         self.assertEqual(t, (147000, 1))
 
 
@@ -1260,7 +1258,6 @@ class SumSpecialValues(NumericTestCase):
         with decimal.localcontext(decimal.BasicContext):
             self.assertRaises(decimal.InvalidOperation, statistics._sum, data)
 
-    @unittest.skipIf(True, "temporarily disabled: see #25928")
     def test_decimal_snan_raises(self):
         # Adding sNAN should raise InvalidOperation.
         sNAN = Decimal('sNAN')