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PEP 736: Final Draft (#4145)

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@ -2,20 +2,21 @@ 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
Discussions-To: https://discuss.python.org/t/pep-736-keyword-argument-shorthand-final-draft/58504
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>`__,
`17-Jul-2024 <https://discuss.python.org/t/pep-736-keyword-argument-shorthand-final-draft/58504>`__,
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)``.
This PEP proposes to introduce syntactic sugar ``f(x=)`` for the common pattern
where a keyword argument has the same name as that of the variable corresponding
to its value ``f(x=x)``.
Motivation
==========
@ -32,25 +33,25 @@ Consider the following call:
)
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.
prevalent among Python libraries. This 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.
keyword. By being explicit, keyword arguments increase readability and
minimise the risk of inadvertent transposition. On the flipside, positional
arguments are often preferred 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:
would confer numerous benefits:
Encourages use of named arguments
---------------------------------
This syntax would encourage the use of named arguments, thereby increasing
By reducing the visual noise that established keyword argument syntax can cause,
this syntax would encourage the use of named arguments, thereby increasing
readability and reducing bugs from argument transposition.
Reduces verbosity
@ -65,7 +66,7 @@ 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.
consistency of variable names used and hence improve readability.
Highlights arguments not following this pattern
-----------------------------------------------
@ -104,7 +105,7 @@ 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.
question beyond the scope of this PEP.
Specification
=============
@ -168,11 +169,11 @@ sometimes referred to as 'punning'. For example:
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>`__.
`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>`__.
`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>`__.
`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
=============
@ -181,7 +182,8 @@ 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 number of keyword arguments which 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
@ -192,7 +194,7 @@ 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_>`__
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%
@ -216,8 +218,8 @@ 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.
``f(x=)`` or ``f(=x)`` being by far the most commonly suggested syntax [1]_
[2]_ [6]_. This strongly indicates that it is the most 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
@ -253,11 +255,11 @@ 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=}"``.
an argument followed only 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 to 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.
@ -270,7 +272,7 @@ f-string syntax as well as similar features in other languages (see
Rejected Ideas
==============
Many alternative syntaxes have been proposed however no syntax other than
Many alternative syntaxes have been proposed however no form 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.
@ -297,7 +299,7 @@ However, we object that:
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
* The use of ``*`` in function calls is well 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)``.
@ -310,7 +312,7 @@ In favour of this form:
``**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
equals 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:
@ -322,7 +324,9 @@ On the contrary:
* 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)``.
* It is less obvious that arbitrary expressions are invalid, for example,
``f(=a + b)``, since such expressions are acceptable after the equals sign in
the current keyword argument syntax but not before it.
``f(%x)`` or ``f(:x)`` or ``f(.x)``
-----------------------------------
@ -344,7 +348,7 @@ 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 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.
@ -358,8 +362,9 @@ 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.
* The expansion of ``x=`` to ``x=x`` is a trivial feature and inherently
significantly less complex than the popular ``*arg`` and ``**kwarg``
expansions.
* This particular syntactic form has been independently proposed on numerous
occasions, indicating that it is the most obvious [1]_ [2]_ [6]_.
@ -379,9 +384,10 @@ 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.
Moreover, by removing the barrier of visual noise introduced by the existing
keyword argument syntax, this syntactic sugar will 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
--------------------------------------------
@ -394,15 +400,15 @@ 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.
A ``NameError`` would make the mistake clear in the large majority 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 (although arguably,
this syntactic sugar could make it harder to spot). Moreover, having variables
with the same name in different scopes is broadly considered to be bad practice
and is discouraged by linters.
Code editors could highlight the issue based on static analysis - ``f(x=)`` is
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.
@ -411,28 +417,28 @@ 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.
and its value have the same semantics in both contexts, that suggests that using
this 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.
However, if the two variables have different semantics, that suggests that this
feature should not be used (since it encourages consistency) or perhaps that one
or both of the variables should be renamed.
Recommendations for using this syntax
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.
applicable, such as via lint rules or style guides.
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.
As described in `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
Impact on Editing
=================
Using a plain text editor
@ -442,7 +448,7 @@ 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
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
@ -450,15 +456,15 @@ 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>`__.
value, a ``NameError`` will typically be raised as described in
`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.
IDEs could handle this syntax. However, we defer to the domain experts
developing IDEs to use their 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.
@ -468,9 +474,9 @@ 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
``f(x=x)`` to identify and highlight cases where the elided 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
@ -510,18 +516,20 @@ 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
* Expand to use the full syntax to pass the unchanged variable as the value of
the renamed argument
* 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.
The last option seems to be the most preferable to reduce unintentional
desynchronisation of names while highlighting the changes to the programmer.
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
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.
Otherwise the AST will remain unchanged.
References
==========