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