Formatter: Don't reorder parameters in function calls

[konstin]

Currently, we format

f(a=b, *args, **kwargs)

as

f(*args, a=b, **kwargs)

Instead, we should preserve the function order.

[NeilGirdhar]

Why do you think that's better? The original order was contentious when we implemented PEP 448. And there's one good reason to do the reorder:

def g():
    print("g")
    return range(2)
def h():
    print("h")
    return 1

def f(*args, **kwargs):
    pass

f(a=h(), *g(), **{})  # Prints g then h!

Positional arguments are evaluated first.

If anything, you should probably add a Ruff rule to warn on keyword arguments being first especially if they have any side effects. The only problem with warning unconditionally is this pattern:

f(x=1, y=2, *args, **kwargs)

[charliermarsh]

I believe we do warn when keyword arguments are provided in such an order: https://beta.ruff.rs/docs/rules/star-arg-unpacking-after-keyword-arg/ (but it's unconditional)

[MichaReiser]

This could be hard with an AST-based approach (assuming I understand the problem correctly).

@NeilGirdhar The main reason for preserving the order is that this change goes beyond just formatting your code. It re-orders code (and potentially re-orders comments too). This is a good case for a lint rule that you can use in addition to the formatter.

[NeilGirdhar]

@charliermarsh My mistake, I tested it but forgot to enable all the rules 😄

@MichaReiser Fair enough, sorry for the noise!

[charliermarsh]

@NeilGirdhar - You're telling me you haven't memorized every single rule in Ruff?

[charliermarsh]

I honestly didn't realize that the evaluation order was independent of the literal order. I should update that rule documentation to include this, it's interesting.

[NeilGirdhar]

Good idea.

It's one of the few places I think. Really glad to see you have rule B026. Definitely an important one for preventing bugs.

Do you have statistics on how often your various rules are hit in public code? If this rule is hit very rarely, we may be able to convince the Python people to syntax-warn, and eventually deprecate this form.