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Python Enhancement Proposals

PEP 736 – Shorthand syntax for keyword arguments at invocation

Joshua Bambrick <jbambrick at>, Chris Angelico <rosuav at>
Discourse thread
Standards Track
14-Oct-2023, 17-Jan-2024

Table of Contents


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).


Keyword argument syntax can become needlessly repetitive and verbose.

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.


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:


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


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.


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:


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.

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}' (see related GitHub issue).

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

Beyond function invocation specifically, more languages offer similar features:


We analysed popular Python libraries from the last few years 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.

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 fastapi rich httpx
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

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 [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 (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 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(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 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 inconsistent but if not, then an arbitrary grouping is enforced between different types of keyword arguments and reordering would be necessary if only one name 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).


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).


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, 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.

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.

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 for cpython has been provided by @Hels15.



Last modified: 2024-05-13 11:02:12 GMT