Merged revisions 71772 via svnmerge from
svn+ssh://pythondev@svn.python.org/python/trunk
........
r71772 | mark.dickinson | 2009-04-20 22:13:33 +0100 (Mon, 20 Apr 2009) | 5 lines
Issue #3166: Make long -> float (and int -> float) conversions
correctly rounded, using round-half-to-even. This ensures that the
value of float(n) doesn't depend on whether we're using 15-bit digits
or 30-bit digits for Python longs.
........
diff --git a/Lib/test/test_long.py b/Lib/test/test_long.py
index 92285b2..53d3e6b 100644
--- a/Lib/test/test_long.py
+++ b/Lib/test/test_long.py
@@ -620,6 +620,65 @@
else:
self.assertRaises(TypeError, pow,longx, longy, int(z))
+ @unittest.skipUnless(float.__getformat__("double").startswith("IEEE"),
+ "test requires IEEE 754 doubles")
+ def test_float_conversion(self):
+ import sys
+ DBL_MAX = sys.float_info.max
+ DBL_MAX_EXP = sys.float_info.max_exp
+ DBL_MANT_DIG = sys.float_info.mant_dig
+
+ exact_values = [0, 1, 2,
+ 2**53-3,
+ 2**53-2,
+ 2**53-1,
+ 2**53,
+ 2**53+2,
+ 2**54-4,
+ 2**54-2,
+ 2**54,
+ 2**54+4]
+ for x in exact_values:
+ self.assertEqual(float(x), x)
+ self.assertEqual(float(-x), -x)
+
+ # test round-half-even
+ for x, y in [(1, 0), (2, 2), (3, 4), (4, 4), (5, 4), (6, 6), (7, 8)]:
+ for p in range(15):
+ self.assertEqual(int(float(2**p*(2**53+x))), 2**p*(2**53+y))
+
+ for x, y in [(0, 0), (1, 0), (2, 0), (3, 4), (4, 4), (5, 4), (6, 8),
+ (7, 8), (8, 8), (9, 8), (10, 8), (11, 12), (12, 12),
+ (13, 12), (14, 16), (15, 16)]:
+ for p in range(15):
+ self.assertEqual(int(float(2**p*(2**54+x))), 2**p*(2**54+y))
+
+ # behaviour near extremes of floating-point range
+ int_dbl_max = int(DBL_MAX)
+ top_power = 2**DBL_MAX_EXP
+ halfway = (int_dbl_max + top_power)//2
+ self.assertEqual(float(int_dbl_max), DBL_MAX)
+ self.assertEqual(float(int_dbl_max+1), DBL_MAX)
+ self.assertEqual(float(halfway-1), DBL_MAX)
+ self.assertRaises(OverflowError, float, halfway)
+ self.assertEqual(float(1-halfway), -DBL_MAX)
+ self.assertRaises(OverflowError, float, -halfway)
+ self.assertRaises(OverflowError, float, top_power-1)
+ self.assertRaises(OverflowError, float, top_power)
+ self.assertRaises(OverflowError, float, top_power+1)
+ self.assertRaises(OverflowError, float, 2*top_power-1)
+ self.assertRaises(OverflowError, float, 2*top_power)
+ self.assertRaises(OverflowError, float, top_power*top_power)
+
+ for p in range(100):
+ x = 2**p * (2**53 + 1) + 1
+ y = 2**p * (2**53 + 2)
+ self.assertEqual(int(float(x)), y)
+
+ x = 2**p * (2**53 + 1)
+ y = 2**p * 2**53
+ self.assertEqual(int(float(x)), y)
+
def test_float_overflow(self):
import math