[Python-Dev] PEP 492 vs. PEP 3152, new round (original) (raw)

Guido van Rossum guido at python.org
Thu Apr 30 00:58:51 CEST 2015

On Wed, Apr 29, 2015 at 3:46 PM, Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:

Yury Selivanov wrote:

I'm not sure why Greg is pushing his Grammar idea so aggressively.

Because I believe that any extra complexity in the grammar needs a very strong justification. It's complexity in the core language, like a new keyword, so it puts a burden on everyone's brain. Saying "I don't think anyone would ever need to write this, therefore we should disallow it" is not enough, given that there is a substantial cost to disallowing it. If you don't think there's a cost, consider that we both seem to be having trouble predicting the consequences of your proposed syntax, and you're the one who invented it. That's not a good sign!

I have a slightly different view. A bunch of things must work, e.g. f(await g(), await h()) or with await f(): (there's a longer list in the PEP). Other things may be ambiguous to most readers, e.g. what does await f() + g() mean, or can we say await await f(), and the solution is to recommend adding parentheses that make things clear to the parser and humans. Yury's proposal satisfies my requirements, and if we really find some unpleasant edge case we can fix it during the 3.5 release (the PEP will be provisional).

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