Module and tests:

* Map conditions to related signals.
* Make contexts unhashable.
* Eliminate used "default" attribute in exception definitions.
* Eliminate the _filterfunc in favor of a straight list.

Docs:
* Eliminate documented references to conditions that are not signals.
* Eliminate parenthetical notes such as "1/0 --> Inf" which are no
  longer true with the new defaults.

Lib/decimal.py

@@ -78,30 +78,30 @@ Traceback (most recent call last):
   ...
 DivisionByZero: x / 0
 >>> c = Context()
->>> c.trap_enablers[DivisionUndefined] = 0
->>> print c.flags[DivisionUndefined]
+>>> c.trap_enablers[InvalidOperation] = 0
+>>> print c.flags[InvalidOperation]
 0
 >>> c.divide(Decimal(0), Decimal(0))
 Decimal("NaN")
->>> c.trap_enablers[DivisionUndefined] = 1
->>> print c.flags[DivisionUndefined]
+>>> c.trap_enablers[InvalidOperation] = 1
+>>> print c.flags[InvalidOperation]
 1
->>> c.flags[DivisionUndefined] = 0
->>> print c.flags[DivisionUndefined]
+>>> c.flags[InvalidOperation] = 0
+>>> print c.flags[InvalidOperation]
 0
 >>> print c.divide(Decimal(0), Decimal(0))
 Traceback (most recent call last):
   ...
   ...
   ...
-DivisionUndefined: 0 / 0
->>> print c.flags[DivisionUndefined]
+InvalidOperation: 0 / 0
+>>> print c.flags[InvalidOperation]
 1
->>> c.flags[DivisionUndefined] = 0
->>> c.trap_enablers[DivisionUndefined] = False
+>>> c.flags[InvalidOperation] = 0
+>>> c.trap_enablers[InvalidOperation] = 0
 >>> print c.divide(Decimal(0), Decimal(0))
 NaN
->>> print c.flags[DivisionUndefined]
+>>> print c.flags[InvalidOperation]
 1
 >>>
 """
@@ -152,7 +152,7 @@ ALWAYS_ROUND = 'ALWAYS_ROUND'  # Every operation rounds at end.
 #Errors
 
 class DecimalException(ArithmeticError):
-    """Base exception class, defines default things.
+    """Base exception class.
 
     Used exceptions derive from this.
     If an exception derives from another exception besides this (such as
@@ -160,12 +160,6 @@ class DecimalException(ArithmeticError):
     called if the others are present.  This isn't actually used for
     anything, though.
 
-    Attributes:
-
-    default -- If the context is basic, the trap_enablers are set to
-               this by default.  Extended contexts start out with them set
-               to 0, regardless.
-
     handle  -- Called when context._raise_error is called and the
                trap_enabler is set.  First argument is self, second is the
                context.  More arguments can be given, those being after
@@ -176,7 +170,6 @@ class DecimalException(ArithmeticError):
     To define a new exception, it should be sufficient to have it derive
     from DecimalException.
     """
-    default = 1
     def handle(self, context, *args):
         pass
 
@@ -288,7 +281,7 @@ class Inexact(DecimalException):
     The inexact signal may be tested (or trapped) to determine if a given
     operation (or sequence of operations) was inexact.
     """
-    default = 0
+    pass
 
 class InvalidContext(InvalidOperation):
     """Invalid context.  Unknown rounding, for example.
@@ -315,7 +308,7 @@ class Rounded(DecimalException):
     The rounded signal may be tested (or trapped) to determine if a given
     operation (or sequence of operations) caused a loss of precision.
     """
-    default = 0
+    pass
 
 class Subnormal(DecimalException):
     """Exponent < Emin before rounding.
@@ -382,19 +375,15 @@ class Underflow(Inexact, Rounded, Subnormal):
     In all cases, Inexact, Rounded, and Subnormal will also be raised.
     """
 
+# List of public traps and flags
+Signals = [Clamped, DivisionByZero, Inexact, Overflow, Rounded,
+           Underflow, InvalidOperation, Subnormal]
 
-def _filterfunc(obj):
-    """Returns true if a subclass of DecimalException"""
-    try:
-        return issubclass(obj, DecimalException)
-    except TypeError:
-        return False
-
-#Signals holds the exceptions
-Signals = filter(_filterfunc, globals().values())
-
-del _filterfunc
-
+# Map conditions (per the spec) to signals
+_condition_map = {ConversionSyntax:InvalidOperation,
+                  DivisionImpossible:InvalidOperation,
+                  DivisionUndefined:InvalidOperation,
+                  InvalidContext:InvalidOperation}
 
 ##### Context Functions #######################################
 
@@ -2168,7 +2157,7 @@ class Context(object):
         return nc
     __copy__ = copy
 
-    def _raise_error(self, error, explanation = None, *args):
+    def _raise_error(self, condition, explanation = None, *args):
         """Handles an error
 
         If the flag is in _ignored_flags, returns the default response.
@@ -2176,6 +2165,7 @@ class Context(object):
         trap_enabler is set, it reaises the exception.  Otherwise, it returns
         the default value after incrementing the flag.
         """
+        error = _condition_map.get(condition, condition)
         if error in self._ignored_flags:
             #Don't touch the flag
             return error().handle(self, *args)
@@ -2183,7 +2173,7 @@ class Context(object):
         self.flags[error] += 1
         if not self.trap_enablers[error]:
             #The errors define how to handle themselves.
-            return error().handle(self, *args)
+            return condition().handle(self, *args)
 
         # Errors should only be risked on copies of the context
         #self._ignored_flags = []
@@ -2207,6 +2197,11 @@ class Context(object):
         for flag in flags:
             self._ignored_flags.remove(flag)
 
+    def __hash__(self):
+        """A Context cannot be hashed."""
+        # We inherit object.__hash__, so we must deny this explicitly
+        raise TypeError, "Cannot hash a Context."
+
     def Etiny(self):
         """Returns Etiny (= Emin - prec + 1)"""
         return int(self.Emin - self.prec + 1)