On Sun, Oct 31, 2021, 6:11 PM Chris Angelico > On Mon, Nov 1, 2021 at 8:56 AM David Mertz, Ph.D. <david.me...@gmail.com> > wrote: > > I am not on the SC, so indeed I won't make the decision. But I *will* > continue to teach Python. New syntax adds a burden to learners, and it > should not be introduced without sufficient benefit to merit that burden. > This proposal does not come close to that threshold. > > How do you currently teach about mutable argument defaults? With this > proposal, it will become trivially easy to have an argument default that's > evaluated fresh every time. Does that not count as beneficial? >
I teach folks to use a sentinel. Yes, it is genuinely a thing to learn, but it takes far less mental effort than special syntax and a different evaluation model. At least 99% of the time, the None sentinel is fine and the best choice.... Yes, I know there are RARE cases where None isn't a good sentinel, but I can't recall the last time I encountered that situation. I've myself made errors with mutable defaults. Perhaps not for a few years, but certainly years after I should have known better. Yes, there genuinely is a possible bug with using defaults. However, I believe that having two different kinds of default parameter bindings would lead to a much larger number of bugs than the status quo. I think this would be true even if the syntax made the distinction obvious... And the '=>' syntax is far from intuitive to start with. It *could* be memorized, but it's definitely not intuitive. This isn't just beginners either. For example, in a few days, I'm giving a short talk about the pitfalls of using lru_cache with mutable arguments to folks at my work. They have, typically, 5-10 years Python experience, yet that's an error I've found in production code. This really isn't the same issue as this PEP, but it's an example of where just a little extra complexity gets experienced developers confused. >
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