Fix a performance issue in Decimal.pow. Thanks Stefan Krah for finding this.

3 files changed, 25 insertions, 6 deletions

diff --git a/Lib/decimal.py b/Lib/decimal.py
index 828027c..71408a8 100644
--- a/Lib/decimal.py
+++ b/Lib/decimal.py
@@ -2047,12 +2047,14 @@ class Decimal(object):
         # case where xc == 1: result is 10**(xe*y), with xe*y
         # required to be an integer
         if xc == 1:
-            if ye >= 0:
-                exponent = xe*yc*10**ye
-            else:
-                exponent, remainder = divmod(xe*yc, 10**-ye)
-                if remainder:
-                    return None
+            xe *= yc
+            # result is now 10**(xe * 10**ye);  xe * 10**ye must be integral
+            while xe % 10 == 0:
+                xe //= 10
+                ye += 1
+            if ye < 0:
+                return None
+            exponent = xe * 10**ye
             if y.sign == 1:
                 exponent = -exponent
             # if other is a nonnegative integer, use ideal exponent
diff --git a/Lib/test/decimaltestdata/extra.decTest b/Lib/test/decimaltestdata/extra.decTest
index 2640842..fce8435 100644
--- a/Lib/test/decimaltestdata/extra.decTest
+++ b/Lib/test/decimaltestdata/extra.decTest
@@ -213,7 +213,20 @@ extr1658 shift 1234567 3 -> 7000
 extr1659 shift 1234567 4 -> 0
 extr1660 shift 1234567 5 -> NaN Invalid_operation
 
+-- Cases where the power function was impossibly slow to determine that the
+-- result is inexact.  Thanks Stefan Krah for identifying this problem.
+precision: 16
+maxExponent: 999999999
+minExponent: -999999999
+extr1700 power 10 1e-999999999 -> 1.000000000000000 Inexact Rounded
+extr1701 power 100.0 -557.71e-742888888 -> 1.000000000000000 Inexact Rounded
+extr1702 power 10 1e-100 -> 1.000000000000000 Inexact Rounded
 
+-- A couple of interesting exact cases for power.  Note that the specification
+-- requires these to be reported as Inexact.
+extr1710 power 1e375 56e-3 -> 1.000000000000000E+21 Inexact Rounded
+extr1711 power 10000 0.75 -> 1000.000000000000 Inexact Rounded
+extr1712 power 1e-24 0.875 -> 1.000000000000000E-21 Inexact Rounded
 
 -- Tests for the is_* boolean operations
 precision: 9
diff --git a/Misc/NEWS b/Misc/NEWS
index 4ec9ec9..369eaff 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -470,6 +470,10 @@ C-API
 Library
 -------
 
+- Fix extreme speed issue in Decimal.pow when the base is an exact
+  power of 10 and the exponent is tiny (for example,
+  Decimal(10) ** Decimal('1e-999999999')).
+
 - Issue #9186: Fix math.log1p(-1.0) to raise ValueError, not OverflowError.
 
 - Issue #9130: Fix validation of relative imports in parser module.