Issue #8567: Fix incorrect precedence of signals in Decimal module.

When a Decimal operation raises multiple signals and more than one of
those signals is trapped, the specification determines the order in
which the signals should be handled.  In many cases this order wasn't
being followed, leading to the wrong Python exception being raised.
This commit fixes those cases, and adds extra tests.  The tests are
only enabled when EXTENDEDERRORTESTS is True, since they involve
rerunning each Decimal testcase several times.

1 files changed, 25 insertions, 0 deletions

diff --git a/Lib/test/test_decimal.py b/Lib/test/test_decimal.py
index 9f3d104e5d..c9a096a86d 100644
--- a/Lib/test/test_decimal.py
+++ b/Lib/test/test_decimal.py
@@ -42,6 +42,12 @@ except ImportError:
 # Useful Test Constant
 Signals = tuple(getcontext().flags.keys())
 
+# Signals ordered with respect to precedence: when an operation
+# produces multiple signals, signals occurring later in the list
+# should be handled before those occurring earlier in the list.
+OrderedSignals = (Clamped, Rounded, Inexact, Subnormal,
+                  Underflow, Overflow, DivisionByZero, InvalidOperation)
+
 # Tests are built around these assumed context defaults.
 # test_main() restores the original context.
 def init():
@@ -352,6 +358,25 @@ class DecimalTest(unittest.TestCase):
                 else:
                     self.fail("Did not raise %s in %s" % (error, s))
                 self.context.traps[error] = 0
+
+            # as above, but add traps cumulatively, to check precedence
+            ordered_errors = [e for e in OrderedSignals if e in theirexceptions]
+            for error in ordered_errors:
+                self.context.traps[error] = 1
+                try:
+                    funct(*vals)
+                except error:
+                    pass
+                except Signals, e:
+                    self.fail("Raised %s in %s; expected %s" %
+                              (type(e), s, error))
+                else:
+                    self.fail("Did not raise %s in %s" % (error, s))
+            # reset traps
+            for error in ordered_errors:
+                self.context.traps[error] = 0
+
+
         if DEBUG:
             print "--", self.context
         try: