[Python-checkins] peps: Elaborate transaction() manager example (Chris A).

Sat Nov 29 02:37:14 CET 2014

https://hg.python.org/peps/rev/4803b0829533
changeset: 5624:4803b0829533
user: Guido van Rossum <guido at python.org>
date: Fri Nov 28 17:26:33 2014 -0800
summary:
 Elaborate transaction() manager example (Chris A).
files:
 pep-0479.txt | 31 ++++++++++++++++++++++---------
 1 files changed, 22 insertions(+), 9 deletions(-)

diff --git a/pep-0479.txt b/pep-0479.txt
--- a/pep-0479.txt
+++ b/pep-0479.txt
@@ -44,26 +44,39 @@
 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()
- yield
- finishing_touches()
+ print('Refactored preparations')
+ yield # Body of with-statement is executed here
+ 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
-normally, causing the failed transaction to be committed! Similarly
+current behavior this exception will be swallowed by the context
+manager; and, worse, the finalization is silently skipped! Similarly
 problematic behavior occurs when an ``asyncio`` coroutine raises
 ``StopIteration``, causing it to terminate silently.
 
-- 
Repository URL: https://hg.python.org/peps
