Add a transaction context manager

[Oliver Beattie]

For users who use transactions heavily, it is sometimes desirable to be able to wrap parts of a function (but not all of it) in a transaction of its own. Previously, the best way to do this was either to do it manually with a try…except…else block or creating a sub-function wrapped in commit_on_success, but context managers in Python 2.5+ can make this much cleaner by just saying…

with transaction:
    # do stuff

…which will handle committing and rolling back. Attached is a patch that I'd love to see implemented in Django :) The function is extremely similar to commit_on_success (the structure is identical).

[Oliver Beattie]

Adds a transaction context manager

[Eric Holscher]

Seems like something that couldn't be supported until 1.3 at least (or whenever we drop 2.4 support).

[Luke Plant]

Actually, defining a context manager does not need the 'with' statement, so it is perfectly possible to add this, providing you only define it is 'contextlib' can be imported (which isn't true for Python < 2.5). Of course, only people running Python 2.5 or later would be able to make use of this. There is the problem of writing tests, but these could either be written explicitly using the magic methods __enter__, __exit__ etc., or they could be omitted since this stuff is pretty well covered by the tests for commit_on_success.

[Oliver Beattie]

This can be pretty easily rewritten as you say with __enter__ and __exit__, thereby circumventing the need for contextlib. I'll update the patch.

[Oliver Beattie]

Second patch with support for <Python 2.5

[Luke Plant]

I was actually referring to writing the tests by calling __enter__() etc. explicitly, but your change is probably a good idea anyway. It still is not compatible with Python 2.3, due to decorator syntax, which is trivial to fix.

[Oliver Beattie]

Oh I see. I don't think it would that big of a deal in that regard — it would require some fiddling around when passing errors to __exit__, but it's really a simple case of doing __exit__(*sys.exc_info()), so not that kludgy.

[Oliver Beattie]

Python 2.3 compatibility

[Alex Gaynor]

(In [14288]) Fixed #10771 -- added support for using the transaction management functions as context managers in Python 2.5 and above. Thanks to Jacob for help with the docs.

[Jacob]

Milestone 1.3 deleted