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.

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 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).

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.

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.