Updates to the with-statement:

- New semantics for __exit__() -- it must re-raise the exception
  if type is not None; the with-statement itself doesn't do this.
  (See the updated PEP for motivation.)

- Added context managers to:
  - file
  - thread.LockType
  - threading.{Lock,RLock,Condition,Semaphore,BoundedSemaphore}
  - decimal.Context

- Added contextlib.py, which defines @contextmanager, nested(), closing().

- Unit tests all around; bot no docs yet.

Lib/decimal.py

@@ -2173,6 +2173,32 @@ for name in rounding_functions:
 
 del name, val, globalname, rounding_functions
 
+class ContextManager(object):
+    """Helper class to simplify Context management.
+
+    Sample usage:
+
+    with decimal.ExtendedContext:
+        s = ...
+    return +s # Convert result to normal precision
+
+    with decimal.getcontext() as ctx:
+        ctx.prec += 2
+        s = ...
+    return +s
+
+    """
+    def __init__(self, new_context):
+        self.new_context = new_context
+    def __enter__(self):
+        self.saved_context = getcontext()
+        setcontext(self.new_context)
+        return self.new_context
+    def __exit__(self, t, v, tb):
+        setcontext(self.saved_context)
+        if t is not None:
+            raise t, v, tb
+
 class Context(object):
     """Contains the context for a Decimal instance.
 
@@ -2224,6 +2250,9 @@ class Context(object):
         s.append('traps=[' + ', '.join([t.__name__ for t, v in self.traps.items() if v]) + ']')
         return ', '.join(s) + ')'
 
+    def __context__(self):
+        return ContextManager(self.copy())
+
     def clear_flags(self):
         """Reset all flags to zero"""
         for flag in self.flags: