[Python-Dev] Can Python guarantee the order of keyword-only parameters? (original) (raw)

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.

/arry -------- 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.covarnames. 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.covarnames 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. /arry ++ Yes, that means "Argument Clinic" should really have been called "Parameter Clinic". But the "Parameter Clinic" sketch is nowhere near as funny.

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