<div dir="ltr">Sure. I think it's a good idea to make this a guaranteed language behavior, and it doesn't need a PEP.<br></div><div class="gmail_extra"><br><div class="gmail_quote">On Sun, Dec 10, 2017 at 1:52 PM, Larry Hastings <span dir="ltr"><<a href="mailto:larry@hastings.org" target="_blank">larry@hastings.org</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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<div class="m_-7947368193309189741moz-forward-container">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.<br>
<br>
<br>
<i>/arry</i><div><div class="h5"><br>
<br>
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<td>Can Python guarantee the order of keyword-only
parameters?</td>
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<th valign="BASELINE" nowrap align="RIGHT">Date: </th>
<td>Mon, 27 Nov 2017 09:05:57 -0800</td>
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<th valign="BASELINE" nowrap align="RIGHT">From: </th>
<td>Larry Hastings <a class="m_-7947368193309189741moz-txt-link-rfc2396E" href="mailto:larry@hastings.org" target="_blank"><larry@hastings.org></a></td>
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<th valign="BASELINE" nowrap align="RIGHT">To: </th>
<td>Python-Dev <a class="m_-7947368193309189741moz-txt-link-rfc2396E" href="mailto:python-dev@python.org" target="_blank"><python-dev@python.org></a></td>
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<br>
<br>
<br>
<br>
First, a thirty-second refresher, so we're all using the same
terminology:<br>
<blockquote>A *parameter* is a declared input variable to a
function.<br>
An *argument* is a value passed into a function. (*Arguments*
are stored in *parameters.*)<br>
<br>
So in the example "def foo(clonk): pass; foo(3)", clonk is a
parameter, and 3 is an argument. ++<br>
</blockquote>
<br>
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.<br>
<br>
But that's arguments. What about parameters?<br>
<br>
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.<br>
<br>
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.<br>
<br>
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.<br>
<br>
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.<br>
<br>
Does this sound reasonable? Would it need a PEP? I'm hoping for
"yes" and "no", respectively.<br>
<br>
<br>
Three final notes:<br>
<ul>
<li>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.</li>
<li>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.<br>
</li>
<li>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.<br>
</li>
</ul>
<br>
<br>
<i>/arry</i><br>
<br>
++ Yes, that means "Argument Clinic" should really have been
called "Parameter Clinic". But the "Parameter Clinic" sketch is
nowhere near as funny. </div></div></div>
</div>
</blockquote></div><br><br clear="all"><br>-- <br><div class="gmail_signature" data-smartmail="gmail_signature">--Guido van Rossum (<a href="http://python.org/~guido" target="_blank">python.org/~guido</a>)</div>
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