Elaborate transaction() manager example (Chris A).

pep-0479.txt

@@ -44,26 +44,39 @@ limitation, but notes that "use cases for these [are] rare to non-
 existent".  Unfortunately while intentional use is rare, it is easy to
 stumble on these cases by accident::
 
+    import contextlib
+ 
     @contextlib.contextmanager
     def transaction():
-        begin()
+        print('begin')
         try:
             yield from do_it()
         except:
-            rollback()
+            print('rollback')
             raise
         else:
-            commit()
+            print('commit')
-    
+ 
     def do_it():
-        initial_preparations()
+        print('Refactored preparations')
-        yield
+        yield # Body of with-statement is executed here
-        finishing_touches()
+        print('Refactored finalization')
+
+    def gene():
+        for i in range(2):
+            with transaction():
+                yield i
+                # return
+                raise StopIteration  # This is wrong
+            print('Should not be reached')
+ 
+    for i in gene():
+        print('main: i =', i)
 
 Here factoring out ``do_it`` into a subgenerator has introduced a
 subtle bug: if the wrapped block raises ``StopIteration``, under the
-current behavior ``do_it`` will fail but report success by returning
+current behavior this exception will be swallowed by the context
-normally, causing the failed transaction to be committed!  Similarly
+manager; and, worse, the finalization is silently skipped!  Similarly
 problematic behavior occurs when an ``asyncio`` coroutine raises
 ``StopIteration``, causing it to terminate silently.