PEP???? Keyword only argument on function call

gistfile1.txt

PEP:
Title: Keyword only argument on function call
Author: Anders Hovmöller <anders.hovmoller@trioptima.com>,
        Johan Lübcke <johan.lubcke@trioptima.com>
Status: Active
Type: Process
Content-Type: text/x-rst
Created: 23-Aug-2018
Post-History: ???


Abstract
========

This PEP proses an additional optional syntax for function calls that
can make keyword argument application shorter and more readable.

Rationale
=========

Keyword arguments increase clarity and make it easier and safer to
refactor code, but there is an incentive to not use them because it's
more to type and more to read. A shorter syntax that improves the
ergonomics of keyword arguments could enable wider use leading to
more robust code.

Specification
=============

The proposed change is rather small: it introduces the * operator as
a valid part of a function call::

    minutes = 5
    seconds = 7
    my_datetime.replace(*, minutes, hours=3, seconds)


The interpretation here is that everything after * must be keyword
arguments, analogous to the existing Python syntax for function
declarations, but here it also implies that if there is no equal sign
for an argument (minutes and seconds above) then this is implicitly
filled in. This means the above call is translated at compile time
to::

    minutes = 5
    seconds = 7
    my_datetime.replace(minutes=minutes, hours=3, seconds=seconds)



More motivating examples
========================

This syntax can make str.format calls nicer. This example from IPython
before::

    warn('Output cache limit (currently {sz} entries) hit.\n'
         'Flushing oldest {cull_count} entries.'.format(sz=sz, cull_count=cull_count))

after::

    warn('Output cache limit (currently {sz} entries) hit.\n'
         'Flushing oldest {cull_count} entries.'.format(*, sz, cull_count))

Another common case is putting objects in a dict, which is very
common when doing web development, before::

    context = dict(user=user, issue=issue, project=project, filters=filters, events=events)

or alternatively::

    context = {'user': user, 'issue': issue, 'filters': filters, 'events': events)

after::

    context = dict(*, user, issue, project, filters, events)

This syntax would also create an incentive to clean up and synchronize
naming in a code base so that you can use the short form instead of the
long form for keyword arguments.

Alternative rejected syntax
===========================

A syntax inspired by OCaml has been previously discussed and been
strongly opposed on the python-ideas mailing list. This syntax was::

    my_datetime.replace(=minutes, hours=3, =seconds)

This alternate syntax is more to type and arguably aesthetically less
pleasing. It also doesn't look like existing Python syntax.

Backwards Compatibility
=======================

The function calling behavior specified in this PEP is a superset
of the existing behavior - that is, it is expected that any
existing programs will continue to work.

Copyright
=========

This document has been placed in the public domain.


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I've posted a tool to analyze how this change could impact a code base: https://gist.github.com/boxed/610b2ba73066c96e9781aed7c0c0b25c for