On Sat, Apr 18, 2020 at 8:18 AM Steven D'Aprano <st...@pearwood.info> wrote:
> This proposal is an alternative to Rodrigo's "Keyword arguments > self-assignment" thread. > > Rodrigo, please feel free to mine this for useful nuggets in your PEP. > > (I don't claim to have invented the syntax -- I think it might have been > Alex Hall?) > lol everyone thinks I invented things I didn't... The idea was first suggested by Dominik: https://mail.python.org/archives/list/python-ideas@python.org/message/S44VDMZ4AFBSGIQEVWMKKWOW4P6WRVXY/ I just suggested making the brackets curly to make it a bit more dict-like. > Keyword Unpacking Shortcut > -------------------------- > > Inside function calls, the syntax > > **{identifier [, ...]} > > expands to a set of `identifier=identifier` argument bindings. > > This will be legal anywhere inside a function call that keyword > unpacking would be legal. That means that syntactically it will legal to > mix this between other keyword arguments: > > # currently legal > func(z=5, **d1, x=0, y=6, **d2, w=7) > > # propose to allow Keyword Unpacking Shortcut as well > func(z=5, **d1, x=0, **{u, v}, y=6, **d2, w=7) > > # which would be equivalent to: > func(z=5, **d1, x=0, u=u, v=v, y=6, **d2, w=7) > My issue with this, and maybe it's what Andrew is also trying to say, is that it breaks our usual assumptions about composing expressions. `{u, v}` is an expression, it represents a set, and it always represents that wherever you put it. Under your proposal, these two programs are both valid syntax with different meanings: f(**{u, v}) x = {u, v} f(**x) Is there anything else similar in the language? Obviously there are cases where the same text has different meanings in different contexts, but I don't think you can ever refactor an expression (or text that looks like an expression) into a variable and change its meaning while keeping the program runnable. This proposal makes it harder for beginners to understand how a program is interpreted. It breaks the simple mental model where building blocks are combined in a consistent fashion into larger parts. The example above looks a bit dumb, but maybe users will try: ``` if flag: kwargs = {u, v} else: kwargs = {w, x} f(**kwargs) ``` Which is valid syntax but is wrong. Then they might try changing that to: f(**({u, v} if flag else {w, x})) which is suddenly invalid syntax. This is a very weird user experience. On that note, is this valid? f(**({u, v}))
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