PEP 736 – Shorthand syntax for keyword arguments at invocation

Author:: Joshua Bambrick <jbambrick at google.com>, Chris Angelico <rosuav at gmail.com>
Sponsor:: Guido van Rossum <guido at python.org>
Discussions-To:: Discourse thread
Status:: Draft
Type:: Standards Track
Created:: 28-Nov-2023
Python-Version:: 3.13
Post-History:: 14-Oct-2023, 17-Jan-2024

Table of Contents

Abstract

This PEP proposes introducing syntactic sugar f(x=) for the common pattern where a named argument is the same as the name of the variable corresponding to its value f(x=x).

Motivation

Keyword argument syntax can become needlessly repetitive and verbose.

Consider the following call:

my_function(
  my_first_variable=my_first_variable,
  my_second_variable=my_second_variable,
  my_third_variable=my_third_variable,
)

The case of a keyword argument name matching the variable name of its value is prevalent among all major Python libraries. This verbosity and redundancy discourages use of named arguments and reduces readability by increasing visual noise.

Rationale

There are two ways to invoke a function with arguments: by position and by keyword. Keyword arguments confer many benefits by being explicit, thus increasing readability and minimising the risk of inadvertent transposition. On the flipside, positional arguments are often used simply to minimise verbosity and visual noise.

We contend that a simple syntactic sugar used to simplify this common pattern which would confer numerous benefits:

Encourages use of named variables

This syntax would encourage the use of named variables, thereby increasing readability (explicit is better than implicit) and reducing bugs from argument transposition.

Reduces verbosity

By minimising visual noise and in some cases lines of code, we can increase readability (readability counts).

Encourages consistent variable names

A common problem is that semantically identical variables have different names depending on their contexts. This syntax would encourage authors to use the same variable name when calling a function as the argument name, which would increase consistency of variable names used and hence also readability.

Applicability to dictionary construction

This syntax can be applied to dictionary construction where a similar pattern frequently occurs (where dictionary keys are identical the names of the variables assigned as their values), {"x": x, "y": y} or dict(x=x, y=y). With this feature, this can now also be trivially written as dict(x=, y=). Whether to further support similar syntax in dictionary literals is an open question out of the scope of this PEP.

Specification

We propose to introduce syntactic sugar such that, if the value of a keyword argument is omitted from a function invocation, the argument’s value is inferred to be the variable matching that name at the invocation scope.

For example, the function invocation:

my_function(my_first_variable=, my_second_variable=, my_third_variable=)

Will be interpreted exactly equivalently to following in existing syntax:

my_function(
  my_first_variable=my_first_variable,
  my_second_variable=my_second_variable,
  my_third_variable=my_third_variable,
)

If no variable matches that name in the invocation scope, a NameError is raised in an identical manner as would be with the established expanded syntax.

This proposal only pertains to function invocations; function defintions are unaffected by the syntax change. All existing valid syntax is unchanged.

Backwards Compatibility

Only new syntax is added which was previously syntactically erroreous. No existing valid syntax is modified. As such, the changes proposed are fully backwards compatible.

Security Implications

There are no security implications for this change.

How to Teach This

Programmers may learn about this feature as an optional abbreviated syntax where keyword arguments are taught. The Python Glossary and Tutorial may be updated accordingly.

Prior Art

Python already possesses a very similar feature in f-string interpolation where f'{x=}' is effectively expanded to f'x={x}' [1].

Several modern languages provide similar features during function invocation, sometimes referred to as ‘punning’. For example:

In Ruby, f(x:, y:) is syntactic sugar for f(x: x, y: y) [2].
In ReasonML, f(~x, ~y) is syntactic sugar for f(~x=x, ~y=y) [3].
In SystemVerilog, (.mult, .mop1, .data); is syntactic sugar for (.mult(mult), .mop1(mop1), .data(data)); [4].

Beyond function invocation specifically, more languages offer similar features:

In OCaml, let+ x in … is syntactic sugar for let+ x = x in … [5].
In JavaScript, { x, y } is syntactic sugar for {x: x, y: y} [6].
In Rust, User { x, y } is shorthand for User {x: x, y: y} [7].

Applicability

We analysed popular Python libraries using this script to compute:

The number of keyword arguments were of the form f(x=x) at invocation.
The percentage of keyword arguments which had the form f(x=x) at invocation.
The number of lines of code which could be saved by using this syntactic sugar to reduce the need for line wraps.

Statistic	cpython	numpy	pandas	scikit-learn
Number of keyword arguments of the form `f(x=x)` at invocation	4,225	2,768	13,235	8,342
Percentage of keyword arguments of the form `f(x=x)` at invocation	11.06%	13.17%	17.24%	18.64%
Lines saved	290	247	935	794

Based on this, we note that the f(x=x) keyword argument pattern is widespread, accounting for 10-20% of all keyword argument uses.

Proposed Syntax

While this feature has been proposed on numerous occasions with several different forms [8] [9] [10] [11] [12], [13] we have opted to advocate for the f(x=) form for the following reasons:

This feature has been proposed frequently over a ten year period with the f(x=) or f(=x) being by far the most common syntax [8] [9] [13]. This is a strong indicator that it is the obvious notation.
The proposed syntax closely matches the f-string debug f'{var=}' syntax (established Pythonic style) and serves an almost identical purpose.
The proposed syntax is exactly analogous to the Ruby keyword argument syntactic sugar [2].
The syntax is easy to implement as it is simple syntactic sugar.
When compared to the prefix form (see Rejected Ideas), this syntax communicates “here is a parameter, go find its argument” which is more appropriate given the semantics of named arguments.
A poll of Python developers indicates that this is the most popular syntax among those proposed.

Rejected Ideas

Many alternative syntaxes have been proposed however no syntax other than f(=x) or f(x=) has garnered significant support. We here enumerate some of the most popular proposed alternatives and why we ultimately reject them.

`f(=x)`

In favour of this form:

The prefix operator is more similar to the established *args and **kwargs syntax for function calls.
It draws more attention to itself when arguments are arranged vertically. In particular, if the arguments are of different lengths it is harder to find the equal sign at the end. Moreover, since Python is read left to right, the use of this feature is clearer to the reader earlier on.

On the contrary:

While the prefix version is visually louder, in practice, there is no need for this feature to shout its presence any more than a typical named argument. By the time we read to the = it is clear that the value is filled in automatically just as the value is clear in the typical keyword argument case.
Semantically, this form communicates ‘here is a value, fill in the parameter’.
which is not what we want to convey.
Less similar to f-string syntax.
Less obvious that arbitrary expressions are invalid, e.g. f(=a+b).

`f(%x)` or `f(:x)` or `f(.x)`

Several flavours of this syntax have been proposed with the prefix form substituting another character for =. However, no such form has gained traction and the choice of symbol seems arbitrary compared to =. Additionally, there is less precedent in terms of existing language features (such as f-string) or other languages (such as Ruby).

`f(a, b, *, x)`

On a few occasions the idea has been floated to borrow the syntax from keyword-only function definitions. This is less arbitrary than f(%x) or variants, but no less so than f(x=).

However, we object that:

For any given argument, it is less clear from local context whether it is positional or named. The * could easily be missed in a long argument list and named arguments may be read as positional or vice versa.
It is unclear whether keyword arguments for which the value was not elided may follow the *. If so, then their relative position will be inconsistent but if not, then an arbitrary grouping is enforced between different types of keyword arguments.

Objections

There are only a few hard objections to the introduction of this syntactic sugar. Most of those not in favour of this feature are in the camp of ‘I wouldn’t use it’. However, over the extensive conversations about this feature, the following objections were the most common:

The syntax is ugly

This objection is by far the most common. On the contrary, we argue that:

This objection is is subjective and many community members disagree.
A nearly-identical syntax is already established for f-strings.
Programmers will, as ever, adjust over time.

The feature is confusing

We argue that:

Introducing new features typically has this impact temporarily.
The syntax is very similar to the established f'{x=}' syntax.
The feature and syntax are familiar from other popular modern languages.
The expansion of x= to x=x is in fact a trivial feature and inherently significantly less complex than *arg and **kwarg expansion.
This particular syntactic form has been independently proposed on numerous occasions, indicating that it is the most obvious [8] [9] [13].

The feature is not explicit

This is based on a misunderstanding of the Zen of Python. Keyword arguments are fundamentally more explicit than positional ones where argument assignment is only visible at the function definition. On the contrary, the proposed syntactic sugar contains all the information as is conveyed by the established keyword argument syntax but without the redundancy. Moreover, the introduction of this syntactic sugar incentivises use of keyword arguments, making typical Python codebases more explicit.

The feature adds another way of doing things

The same argument can be made against all syntax changes. This is a simple syntactic sugar, much as x += 1 is sugar for x = x + 1 when x is an integer. This isn’t tantamount to a ‘new way’ of passing arguments but a more readable notation for the same way.

Renaming the variable in the calling context will break the code

A NameError would make the mistake abundantly clear. Moreover, text editors could highlight this based on static analysis ‒ f(x=) is exactly equivalent to writing f(x=x). If x does not exist, modern editors have no problem highlighting the issue.

Last modified: 2024-03-10 19:52:57 GMT

Python Enhancement Proposals