python-peps/peps/pep-0736.rst

PEP: 736
Title: Shorthand syntax for keyword arguments at invocation
Author: Joshua Bambrick <jbambrick@google.com>,
        Chris Angelico <rosuav@gmail.com>
Discussions-To: https://discuss.python.org/t/pep-736-shorthand-syntax-for-keyword-arguments-at-invocation/43432
Status: Draft
Type: Standards Track
Created: 28-Nov-2023
Python-Version: 3.14
Post-History: `14-Oct-2023 <https://discuss.python.org/t/syntactic-sugar-to-encourage-use-of-named-arguments/36217>`__,
              `17-Jan-2024 <https://discuss.python.org/t/pep-736-shorthand-syntax-for-keyword-arguments-at-invocation/43432>`__,

Abstract
========

This PEP proposes introducing syntactic sugar ``f(x=)`` for the 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 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 arguments
---------------------------------

This syntax would encourage the use of named arguments, thereby increasing
readability and reducing bugs from argument transposition.

Reduces verbosity
-----------------

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

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.

Highlights arguments not following this pattern
-----------------------------------------------

With the current syntax, function calls where many arguments are forwarded from
the local context can make other argument values easy to miss due to the visual
noise. For example::

    add_middleware(
        excluded_urls=excluded_urls,
        server_request=server_request,
        client_request=client_request,
        client_response=client_response,
        span_details=_get_span_details(),
        tracer=tracer,
        meter=meter,
    )

With this syntax, the exceptional arguments become easier to identify::

    add_middleware(
        excluded_urls=,
        server_request=,
        client_request=,
        client_response=,
        span_details=_get_span_details(),
        tracer=,
        meter=,
    )

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 definitions are
unaffected by the syntax change. All existing valid syntax is unchanged.

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

Only new syntax is added which was previously syntactically erroneous. 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.

Prior Art
=========

Python already possesses a very similar feature in f-string interpolation where
``f'{x=}'`` is effectively expanded to ``f'x={x}'`` (see
`related GitHub issue <https://github.com/python/cpython/issues/80998>`__).

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)``. See the
  `Ruby 3.1.0 release notes <https://www.ruby-lang.org/en/news/2021/12/25/ruby-3-1-0-released/#:~:text=Other%20Notable%20New%20Features>`__ (search for "keyword arguments").
* In ReasonML, ``f(~x, ~y)`` is syntactic sugar for ``f(~x=x, ~y=y)``. See the
  `ReasonML function documentation <https://reasonml.github.io/docs/en/function#function-application>`__ (search for "punning").
* In SystemVerilog, ``(.mult, .mop1, .data);`` is syntactic sugar for
  ``(.mult(mult), .mop1(mop1),  .data(data));``. See
  `SystemVerilog Implicit Port Connections <http://www.sunburst-design.com/papers/CummingsDesignCon2005_SystemVerilog_ImplicitPorts.pdf>`__.
* In Jakt, ``f(x, y)`` is syntactic sugar for ``f(x: x, y: y)``. See
  `The Jakt programming language <https://github.com/SerenityOS/jakt?tab=readme-ov-file#function-calls>`__.

Beyond function invocation specifically, more languages offer similar features:

* In OCaml, ``let+ x in …`` is syntactic sugar for ``let+ x = x in …``. See
  `OCaml Short notation for variable bindings (let-punning) <https://v2.ocaml.org/manual/bindingops.html#ss:letops-punning>`__.
* In JavaScript, ``{ x, y }`` is syntactic sugar for ``{x: x, y: y}``. See
  `JavaScript Object Initializer <https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Object_initializer>`__.
* In Rust, ``User { x, y }`` is shorthand for ``User {x: x, y: y}``. See
  `Rust Using the Field Init Shorthand <https://doc.rust-lang.org/book/ch05-01-defining-structs.html#using-the-field-init-shorthand-when-variables-and-fields-have-the-same-name>`__.

Applicability
=============

We analysed popular Python libraries from the last few years using
`this script <https://gist.github.com/joshuabambrick/a850d0e0050129b9252c748fa06c48b2>`__
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.

The purpose of this exercise was to compute statistics about the prevalence of
this pattern and should not be interpreted as a recommendation that the proposed
syntactic sugar should be applied universally.

===================================================================== =============== ================ ============= ==============
Statistic                                                             `polars <a_>`__ `fastapi <b_>`__ `rich <c_>`__ `httpx <d_>`__
===================================================================== =============== ================ ============= ==============
Number of keyword arguments of the form ``f(x=x)`` at invocation      1,654           1,408            566           759
Percentage of keyword arguments of the form ``f(x=x)`` at invocation  15.83%          28.11%           15.74%        45.13%
Lines saved                                                           170             35               62            117
===================================================================== =============== ================ ============= ==============

.. _a: https://github.com/joshuabambrick/polars/pull/1
.. _b: https://github.com/joshuabambrick/fastapi/pull/1
.. _c: https://github.com/joshuabambrick/rich/pull/1
.. _d: https://github.com/joshuabambrick/httpx/pull/1

Based on this, we note that the ``f(x=x)`` keyword argument pattern is
widespread, accounting for anywhere from 15% to just below half of all keyword
argument uses depending on the codebase.

Proposed Syntax
===============

While this feature has been proposed on numerous occasions with several
different forms [1]_ [2]_ [3]_ [4]_ [5]_, [6]_ 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 [1]_ [2]_ [6]_.
  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. See the
  `Ruby 3.1.0 release notes <https://www.ruby-lang.org/en/news/2021/12/25/ruby-3-1-0-released/#:~:text=Other%20Notable%20New%20Features>`__ (search for "keyword arguments").
* 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 <https://discuss.python.org/t/syntactic-sugar-to-encourage-use-of-named-arguments/36217/130>`__
  indicates that this is the most popular syntax among those proposed.

How to Teach This
=================

To ease the communication of and search for this feature, it may also be
valuable to provide this feature with a name, such as 'keyword argument
shorthand'.

Keen Python developers will likely hear about this feature through typical
information channels, such as newsboards, social media, mailing lists, online
forums, or word of mouth. Many more will encounter this feature while reading
code and noting the omission of the value in a keyword argument at invocation,
violating their expectations. We should ensure such developers have easy access
to documentation that explains the semantics of this feature and that this
documentation is easy to find when searching. For example, the
`Python Glossary <https://docs.python.org/3/glossary.html#term-argument>`__ and
`Tutorial <https://docs.python.org/3/tutorial/controlflow.html#keyword-arguments>`__
may be updated accordingly and reasonable keywords may be used to help with
search discoverability.
`A StackOverflow question <https://stackoverflow.blog/2011/07/01/its-ok-to-ask-and-answer-your-own-questions/>`__
could be written to help explain this feature to those searching for an
explanation.

A teacher may explain this feature to new Python programmers as, "where you see
an argument followed by an equals sign, such as ``f(x=)``, this represents a
keyword argument where the name of the argument and its value are the same. This
can be written equivalently in the expanded notation, ``f(x=x)``." Depending on
a student's background, a teacher might further compare this to equivalent
syntax in other languages or Python's f-string syntax ``f"{x=}"``.

To understand this, a student of Python would need to be familiar with the
basics of functions in addition to the existing keyword argument syntax.
Given that this feature is a relatively straightforward syntactic sugar, it is
reasonable that a student who possesses a grasp of keyword arguments will be
able to absorb this concept quickly. This is evidenced by the success of the
f-string syntax as well as similar features in other languages (see
`Prior Art`_).

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

On a few occasions the idea has been floated to borrow the syntax from
keyword-only function definitions.

In favour of this proposal:

* This syntax is familiar from its use to require keyword-only arguments in
  function definitions.
* `A poll of Python developers <https://discuss.python.org/t/syntactic-sugar-to-encourage-use-of-named-arguments/36217/130>`__
  indicates that this is the second most popular syntax among those proposed.

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 confusingly
  arbitrary, but if not, then an arbitrary grouping is enforced between
  different types of keyword arguments and reordering of arguments would be
  necessary if only one name (the argument or its value) was changed.
* The use of ``*`` in function calls is established and this proposal would
  introduce a new effect which could cause confusion. For example,
  ``f(a, *x, y)`` would mean something different than ``f(a, *, x, y)``.

``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.
* It is less similar to f-string syntax.
* It is 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).

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 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 [1]_ [2]_ [6]_.

The feature is not explicit
---------------------------

We recognise that, in an obvious sense, the argument value is 'implicit' in this
proposed syntax. However, we do not think that this is what the Zen of Python is
aiming to discourage.

In the sense that we take the Zen to be referring to, keyword arguments (for
example) are more explicit than positional arguments where the argument name is
omitted and impossible to tell from the local context. Conversely, the syntactic
sugar for integers ``x += 1`` is not more implicit than ``x = x + 1`` in this
sense, even though the variable is omitted from the right hand side, because it
is immediately obvious from the local context what it is.

The syntax proposed in this PEP is much more closely analogous to the ``x += 1``
example (although simpler since we do not propose to introduce a new operation).
Moreover, the introduction of this syntactic sugar should encourage the use of
keyword arguments over positional ones, making typical Python codebases more
explicit in general.

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 clear in most cases. There may be
confusion if a variable from a broader scope has the same name as the original
variable, so no ``NameError`` would be raised. However, this issue can also
occur with keyword arguments using the current syntax (arguably, this syntactic
sugar could make it harder to spot). Moreover, having variables with the same
name in different scopes is broadly considered bad practice and discouraged by
linters.

Code editors could highlight the issue 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.

This syntax increases coupling
------------------------------

We recognise that, as ever, all syntax has the potential for misuse and so
should be applied judiciously to improve codebases. In this case, if a parameter
and its value have the same semantics in both contexts, that may suggest that
using this new syntax is appropriate and will help ameliorate the risk of
unintentional desynchronisation which harms readability.

However, if the two variables have different semantics, that may suggest that
this feature should not be used to encourage consistency or even that they
should be renamed.

Recommendations for using this syntax
=====================================

As with any other language feature, the programmer should exercise their own
judgement about whether it is prudent to use it in any given context. We do not
recommend enforcing a rule to use the feature in all cases where it may be 
applicable.

As described `above <This syntax increases coupling>`__, we propose that a
reasonable rule of thumb would be to use this in cases where a parameter and its
argument have the same semantics in order to reduce unintentional
desynchronisation without causing inappropriate coupling.

Impact on editing
=================

Using a plain text editor
-------------------------

Editing with a plain text editor should generally be unaffected.

When renaming a variable using a 'Find-Replace' method, where this syntax is
used the developer will come across the function argument at invocation (as they
would if this syntax was not used). At that point, they can as usual decide
whether to update the argument as well or expand to the full ``f(x=x)`` syntax.

As with the current syntax, a 'Find-Replace All' method would fail since the
keyword argument would not exist at function definition, in the vast majority
of cases.

If the developer leaves the argument name unchanged and forgets to update its
value, a ``NameError`` will typically be raised as described
`above <Renaming the variable in the calling context will break the code>`__.

Proposals for IDEs
------------------

In response to community feedback, we include some suggestions regarding how
IDEs could handle this syntax. However, we of course defer to the domain experts
developing IDEs to use their own discretion.

Most considerations are made simple by recognising that ``f(x=)`` is just
syntactic sugar for ``f(x=x)`` and should be treated the same as at present.

Highlighting NameErrors
'''''''''''''''''''''''

IDEs typically offer a feature to highlight code that may cause a ``NameError``.
We recommend that this syntax be treated similarly to the expanded form
``f(x=x)`` to identify and highlight cases where the elided value variable may
not exist. What visual cue may be used to highlight these cases may be the same
or different from that which would be used with the current syntax, depending on
the IDE.

Jump to definition
''''''''''''''''''

There are a few possible ways that a 'jump to definition' feature could be
implemented depending on the caret/cursor position.

One option is to:

* Jump to the argument in the function definition if the caret/cursor is on the
  argument
* Jump to the definition of the elided variable if the caret/cursor is on the
  character following the ``=`` in our proposed syntax

Another, potentially complementary, option would be to expand the syntax
visually on mouseover and enable a ``Ctrl+Click`` (or ``Cmd+Click``) to the
definition of the variable.

Highlighting other references
'''''''''''''''''''''''''''''

IDEs frequently highlight matching code references to the value at the current
caret/cursor position. With this shorthand syntax, when the caret/cursor is on
the argument name it may be valuable to either:

* Highlight both references to the argument and its value reflecting the fact
  that this name now refers to both
* Visually expand the syntax on mouseover (as above) and apply established
  highlighting logic according to the cursor

Rename symbol
'''''''''''''

There are a few ways that IDEs may wish to support a 'Rename symbol' feature for
this syntax. For example, if the argument is being renamed, the IDE may:

* Also rename the variable used as its value in each calling context where this
  syntax is used
* Expand to use the full syntax to pass the variable used as its value
* Prompt the developer to select between the two above options

The last option here seems most preferable in order to reduce unintentional
desynchronisation of names while highlighting the user to the changes.

Reference Implementation
========================

`A proposed implementation <https://github.com/Hels15/cpython/tree/last-build>`_
for cpython has been provided by @Hels15. We will extend this implementation to
add an AST node attribute indicating for keywords whether the value was elided.

References
==========

.. [1] Short form for keyword arguments and dicts (2013)
   https://mail.python.org/archives/list/python-ideas@python.org/thread/SQKZ273MYAY5WNIQRGEDLYTKVORVKNEZ/#LXMU22F63VPCF7CMQ4OQRH2CG6H7WCQ6
.. [2] Keyword arguments self-assignment (2020)
   https://mail.python.org/archives/list/python-ideas@python.org/thread/SIMIOC7OW6QKLJOTHJJVNNBDSXDE2SGV/
.. [3] Shorthand notation of dict literal and function call (2020)
   https://discuss.python.org/t/shorthand-notation-of-dict-literal-and-function-call/5697/1
.. [4] Allow identifiers as keyword arguments at function call site (extension
   of PEP 3102?) (2023)
   https://discuss.python.org/t/allow-identifiers-as-keyword-arguments-at-function-call-site-extension-of-pep-3102/31677
.. [5] Shorten Keyword Arguments with Implicit Notation: foo(a=a, b=b) to foo(.a, .b) (2023)
   https://discuss.python.org/t/shorten-keyword-arguments-with-implicit-notation-foo-a-a-b-b-to-foo-a-b/33080
.. [6] Syntactic sugar to encourage use of named arguments (2023)
   https://discuss.python.org/t/syntactic-sugar-to-encourage-use-of-named-arguments/36217

Copyright
=========

This document is placed in the public domain or under the
CC0-1.0-Universal license, whichever is more permissive.