[Python-Dev] We should be using a tool for code reviews
debatem1 at gmail.com
Thu Sep 30 18:53:00 CEST 2010
On Thu, Sep 30, 2010 at 9:33 AM, Barry Warsaw <barry at python.org> wrote:
> On Sep 30, 2010, at 10:47 AM, Jesse Noller wrote:
>>Not to mention; there's a lot to be learned from doing them on both
>>sides. At work, I learn about chunks of code I might not have
>>otherwise known about or approaches to a problem I'd never considered.
>>I sort of drank the kool-aid.
> Tools aside, I completely agree.
> Many projects that I contribute to have policies such as "nothing lands
> without reviewer approval". For some that means one reviewer must approve it,
> for others two +1s and no -1s, or a coding approval and a ui approval, etc.
> When I was the review team lead on Launchpad, I had a goal that every
> developer would also eventually be a reviewer. We started with a small number
> of experienced developers, then ran a mentor program to train new reviewers
> (who we called "mentats"). A mentat approval was not enough to land a branch,
> but the mentor could basically say "yes, i agree with the review" and it would
> land. Eventually, by mutual consent a mentat would graduate to full
> reviewership (and hopefully be a mentor to new reviewers).
> This was hugely successful among many dimensions. It got everyone on the same
> page as to coding standards, it equalized the playing field, it got everyone
> to think collaboratively as a team, folks learned about parts of the system
> they didn't have day-to-day intimate knowledge about, and it got changes
> landed much more quickly.
> Here are a few things we learned along the way. Their applicability to Python
> will vary of course, and we need to find what works for us.
> * Keep branches *small*. We had a limit of 800 lines of diff, with special
> explicit permission from the person reviewing your change to exceed. 800
> lines is about the maximum that a person can review in a reasonable amount
> of time without losing focus.
> * The *goal* was 5 minutes review, but the reality was that most reviews took
> about 15-20 minutes. If it's going longer, you weren't doing it right.
> This meant that there was a level of trust between the reviewer and the dev,
> so that the basic design had been previously discussed and agreed upon (we
> mandated pre-implementation chats), etc. A reviewer was there to sanity
> check the implementation, watch for obvious mistakes, ensure test coverage
> for the new code, etc. If you were questioning the basic design, you
> weren't doing a code review. It was okay to reject a change quickly if you
> found fatal problems.
> * The primary purpose of a code review was to learn and teach, and in a sense,
> the measurable increase in quality was a side-effect. What I mean by that
> is that the review process cannot catch all bugs. It can reduce them, but
> it's much more valuable to share expertise on how to do things. E.g. "Did
> you know that if X happens, you won't be decref'ing Y? We like to use goto
> statements to ensure that all objects are properly refcounted even in the
> case of exceptional conditions." That's a teaching moment that happens to
> improve quality.
> * Reviews are conversations, and it's a two way street. Many times a dev
> pushed back on one of my suggestions, and we'd have weekly reviewer meetings
> to hash out coding standards differences. E.g. Do you check for empty
> sequences with "if len(foo) == 0" or "if not foo"? The team would make
> those decisions and you'd live by them even if you didn't agree. It's also
> critical to document those decisions, and a wiki page style guide works very
> well (especially when you can point to PEP 8 as your basic style guide for
> * Reviews are collaborations. You're both there to get the code landed so
> work together, not at odds. Try to reach consensus, and don't be afraid to
> compromise. All code is ultimately owned by everyone and you never know who
> will have to read it 2 years from now, so keep things simple, clear, and
> well commented. These are all aesthetics that a reviewer can help with.
> * As a reviewer ASK QUESTIONS. The best reviews were the ones that asked lots
> of questions, such as "have you thought about the race conditions this might
> introduce?" or "what happens when foo is None?" A reviewer doesn't
> necessarily have to guess or work out every detail. If you don't understand
> something, ask a question and move on. Let the coder answer it to your
> satisfaction. As a reviewer, once all your questions are answered, you know
> you can approve the change.
> * Keep reviews fast, easy, and fun. I think this is especially true for
> Python, where we're all volunteers. Keeping it fast, easy, and fun greatly
> improves the odds that code will be reviewed for the good of the project.
> * Have a dispute resolution process. If a reviewer and coder can't agree,
> appeal to a higher authority. As review team leader, I did occasionally
> have to break ties.
> * Record the reviewer in the commit messages. We had highly structured commit
> messages that included the reviewer name, e.g.
> % commit -m"[r=barry] Bug 12345; fix bad frobnification in Foo.bar()"
> thus recording in the revision history both the coder and the reviewer, so
> that we could always ask someone about the change.
> * Don't let changes get stale. We had a goal that changes would go from
> ready-to-code (i.e. design and implementation strategy have already been
> worked out) to landed in 2 days. The longer a change goes before landing,
> the more stale it gets, the more conflicts you can have, and the less
> relevant the code becomes.
> I know this sounds like a lot of process, but it really was fairly lightweight
> in practice. And that's the most important! Keep things fast, easy, and fun
> and it'll get done. Also, these ideas evolved after 3 years of
> experimentation with various approaches, so let it take some time to evolve.
> And don't be so married to process that you're afraid to ditch steps that are
> wasteful and don't contribute value to the project.
> Certainly some of our techniques won't be relevant for Python. For example,
> we assigned people to do nothing but reviews for one day out of the week (we
> call it "on-call reviewers"). This worked for us because team velocity was
> much more important than individual velocity. Even though at first blush, it
> seemed like you personally were going to be 20% less productive, team
> productivity skyrocketed because changes were landing much faster, with much
> less waste built in. This probably won't work for Python where our
> involvement is not governed by paycheck, but by the whims of our real jobs and
> life commitments.
Extremely well put. Could this kind of process be put in place for the
code sprints Jesse's interested in? Seems like an ideal testbed.
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