Elaborate Decimal context example, thanks to Michael Chermside.

pep-0343.txt

@@ -388,9 +388,33 @@ Examples
        by default all signals are blocked.  The implementation is left
        as an exercise to the reader.
 
-    8. Another use for this feature is the Decimal context.  It's left
+    8. Another use for this feature is the Decimal context.  Here's a
-       as an exercise for the reader.  (Mail it to me if you'd like to
+       simple example, after one posted by Michael Chermside:
-       see it here.)
+
+        import decimal
+
+        @do_template
+        def with_extra_precision(places=2):
+            c = decimal.getcontext()
+            saved_prec = c.prec
+            c.prec += places
+            yield None
+            c.prec = saved_prec
+
+       Sample usage (adapted from the Python Library Reference):
+
+        def sin(x):
+            "Return the sine of x as measured in radians."
+            do with_extra_precision():
+                i, lasts, s, fact, num, sign = 1, 0, x, 1, x, 1
+                while s != lasts:
+                    lasts = s
+                    i += 2
+                    fact *= i * (i-1)
+                    num *= x * x
+                    sign *= -1
+                    s += num / fact * sign
+                return +s
 
 References