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

Larry Hastings larry at hastings.org
Mon Nov 27 12:05:57 EST 2017

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:

//arry/

++ Yes, that means "Argument Clinic" should really have been called "Parameter Clinic".  But the "Parameter Clinic" sketch is nowhere near as funny. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://mail.python.org/pipermail/python-dev/attachments/20171127/af157416/attachment.html>

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