More generic decimal example.

pep-0343.txt

@@ -418,19 +418,28 @@ Examples
             # so this must be outside the do-statement:
             return +s
 
-       An alternative version of the generator saves the entire
-       context instead of just the precision attribute.  This is more
-       robust, but also more expensive, making it perhaps the better
-       choice when several attributes are modified together:
+    9. Here's a more general Decimal-context-switching template:
 
         @do_template
-        def with_extra_precision(places=2):
-            oldcontext = decimal.getcontext()
-            newcontext = oldcontext.copy()
-            newcontext.prec += places
-            decimal.setcontext(newcontext)
-            yield None
-            decimal.setcontext(oldcontext)
+        def with_decimal_context(newctx=None):
+            oldctx = decimal.getcontext()
+            if newctx is None:
+                newctx = oldctx.copy()
+            decimal.setcontext(newctx)
+            yield newctx
+            decimal.setcontext(oldctx)
+
+       Sample usage (adapted from the previous one):
+
+        def sin(x):
+            do with_decimal_context() as ctx:
+                ctx.prec += 2
+                # Rest of algorithm the same
+            return +s
+
+       (Nick Coghlan has proposed to add __enter__ and __exit__
+       methods to the decimal.Context class so that this example can
+       be simplified to "do decimal.getcontext() as ctx: ...".)
 
 References