[Python-Dev] PEP 492 quibble and request (original) (raw)

Guido van Rossum guido at python.org
Thu Apr 30 04:31:22 CEST 2015

On Wed, Apr 29, 2015 at 7:07 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:

[...] Yeah, I'm coming around to the idea. For the async pseudo-keyword, I can see that the proposal only allows its use in cases that were previously entirely illegal, but I'm not yet clear on how the PEP proposes to avoid changing the meaning of the following code:

x = await(thisisafunctioncall) Unless I'm misreading the proposed grammar in the PEP (which is entirely possible), I believe PEP 492 would reinterpret that as: x = await thisisnotafunctioncallanymore

Ah, but here's the other clever bit: it's only interpreted this way inside a function declared with 'async def'. Outside such functions, 'await' is not a keyword, so that grammar rule doesn't trigger. (Kind of similar to the way that the print_function future disables the keyword-ness of 'print', except here it's toggled on or off depending on whether the nearest surrounding scope is 'async def' or not. The PEP could probably be clearer about this; it's all hidden in the Transition Plan section.)

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