[Python-Dev] Can Python guarantee the order of keyword-only parameters?
Guido van Rossum
guido at python.org
Sun Dec 10 17:20:49 EST 2017
Sure. I think it's a good idea to make this a guaranteed language behavior,
and it doesn't need a PEP.
On Sun, Dec 10, 2017 at 1:52 PM, Larry Hastings <larry at hastings.org> wrote:
> Can I get a ruling on this? I got +1s from the community, but as it's a
> (minor) language thing I feel like you're the only one who can actually
> okay it.
> -------- Forwarded Message --------
> Subject: Can Python guarantee the order of keyword-only parameters?
> Date: Mon, 27 Nov 2017 09:05:57 -0800
> From: Larry Hastings <larry at hastings.org> <larry at hastings.org>
> To: Python-Dev <python-dev at python.org> <python-dev at python.org>
> First, a thirty-second refresher, so we're all using the same terminology:
> A *parameter* is a declared input variable to a function.
> An *argument* is a value passed into a function. (*Arguments* are stored
> in *parameters.*)
> So in the example "def foo(clonk): pass; foo(3)", clonk is a parameter,
> and 3 is an argument. ++
> Keyword-only arguments were conceived of as being unordered. They're
> stored in a dictionary--by convention called **kwargs--and dictionaries
> didn't preserve order. But knowing the order of arguments is occasionally
> very useful. PEP 468 proposed that Python preserve the order of
> keyword-only arguments in kwargs. This became easy with the
> order-preserving dictionaries added to Python 3.6. I don't recall the
> order of events, but in the end PEP 468 was accepted, and as of 3.6 Python
> guarantees order in **kwargs.
> But that's arguments. What about parameters?
> Although this isn't as directly impactful, the order of keyword-only
> parameters *is* visible to the programmer. The best way to see a
> function's parameters is with inspect.signature, although there's also the
> deprecated inspect.getfullargspec; in CPython you can also directly examine
> fn.__code__.co_varnames. Two of these methods present their data in a way
> that preserves order for all parameters, including keyword-only
> parameters--and the third one is deprecated.
> Python must (and does) guarantee the order of positional and
> positional-or-keyword parameters, because it uses position to map arguments
> to parameters when the function is called. But conceptually this isn't
> necessary for keyword-only parameters because their position is
> irrelevant. I only see one place in the language & library that addresses
> the ordering of keyword-only parameters, by way of omission. The PEP for
> inspect.signature (PEP 362) says that when comparing two signatures for
> equality, their positional and positional-or-keyword parameters must be in
> the same order. It makes a point of *not* requiring that the two
> functions' keyword-only parameters be in the same order.
> For every currently supported version of Python 3, inspect.signature and
> fn.__code__.co_varnames preserve the order of keyword-only parameters.
> This isn't surprising; it's basically the same code path implementing those
> as the two types of positional-relevant parameters, so the most
> straightforward implementation would naturally preserve their order. It's
> just not guaranteed.
> I'd like inspect.signature to guarantee that the order of keyword-only
> parameters always matches the order they were declared in. Technically
> this isn't a language feature, it's a library feature. But making this
> guarantee would require that CPython internally cooperate, so it's kind of
> a language feature too.
> Does this sound reasonable? Would it need a PEP? I'm hoping for "yes"
> and "no", respectively.
> Three final notes:
> - Yes, I do have a use case. I'm using inspect.signature metadata to
> mechanically map arguments from an external domain (command-line arguments)
> to a Python function. Relying on the declaration order of keyword-only
> parameters would elegantly solve one small problem.
> - I asked Armin Rigo about PyPy's support for Python 3. He said it
> should already maintain the order of keyword-only parameters, and if I ever
> catch it not maintaining them in order I should file a bug. I assert that
> making this guarantee would be nearly zero effort for any Python
> implementation--I bet they all already behave this way, all they need is a
> test case and some documentation.
> - One can extend this concept to functools.partial and
> inspect.Signature.bind: should its transformations of keyword-only
> parameters also maintain order in a consistent way? I suspect the answer
> there is much the same--there's an obvious way it should behave, it almost
> certainly already behaves that way, but it doesn't guarantee it. I don't
> think I need this for my use case.
> ++ Yes, that means "Argument Clinic" should really have been called
> "Parameter Clinic". But the "Parameter Clinic" sketch is nowhere near as
--Guido van Rossum (python.org/~guido)
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