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PEP 570 -- Python Positional-Only Parameters

Title:Python Positional-Only Parameters
Author:Larry Hastings <larry at>, Pablo Galindo <pablogsal at>, Mario Corchero <mariocj89 at>
Discussions-To:Python-Dev <python-dev at>
Type:Standards Track


This PEP proposes a syntax for positional-only parameters in Python. Positional-only parameters are parameters without an externally-usable name; when a function accepting positional-only parameters is called, positional arguments are mapped to these parameters based solely on their position.


Python has always supported positional-only parameters. Early versions of Python lacked the concept of specifying parameters by name, so naturally all parameters were positional-only. This changed around Python 1.0, when all parameters suddenly became positional-or-keyword. This allowed users to provide arguments to a function both positionally or referencing the keyword used in the definition of it. But, this is not always desired nor even available as even in current versions of Python, many CPython "builtin" functions still only accept positional-only arguments.

Even if positional arguments only in a function can be achieved via using *args parameters and extracting them one by one, the solution is far from ideal and not as expressive as the one proposed in this PEP, which targets to provide syntax to specify accepting a specific number of positional-only parameters. Additionally, this will bridge the gap we currently find between builtin functions that today allows to specify positional-only parameters and pure Python implementations that lack the syntax for it.

Positional-Only Parameter Semantics In Current Python

There are many, many examples of builtins that only accept positional-only parameters. The resulting semantics are easily experienced by the Python programmer--just try calling one, specifying its arguments by name:

>>> help(pow)
pow(x, y, z=None, /)
>>> pow(x=5, y=3)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: pow() takes no keyword arguments

Pow clearly expresses that its arguments are only positional via the / marker, but this is at the moment only documentational, Python developers cannot write such syntax.

In addition, there are some functions with particularly interesting semantics:

  • range(), which accepts an optional parameter to the left of its required parameter. [2]
  • dict(), whose mapping/iterator parameter is optional and semantically must be positional-only. Any externally visible name for this parameter would occlude that name going into the **kwarg keyword variadic parameter dict! [1]

Obviously one can simulate any of these in pure Python code by accepting (*args, **kwargs) and parsing the arguments by hand. But this results in a disconnect between the Python function signature and what it actually accepts, not to mention the work of implementing said argument parsing.


The new syntax will allow developers to further control how their API can be consumed. It will allow restricting the usage of keyword Specify arguments by adding the new type of positional-only ones.

A similar PEP with a broader scope (PEP 457) was proposed to define the syntax. This PEP builds on top of part of it to define and provide an implementation for the / syntax on function signatures.

The Current State Of Documentation For Positional-Only Parameters

The documentation for positional-only parameters is incomplete and inconsistent:

  • Some functions denote optional groups of positional-only arguments by enclosing them in nested square brackets. [3]
  • Some functions denote optional groups of positional-only arguments by presenting multiple prototypes with varying numbers of arguments. [4]
  • Some functions use both of the above approaches. [2] [5]

One more important idea to consider: currently in the documentation there's no way to tell whether a function takes positional-only parameters. open() accepts keyword arguments, ord() does not, but there is no way of telling just by reading the documentation that this is true.

Syntax And Semantics

From the "ten-thousand foot view", and ignoring *args and **kwargs for now, the grammar for a function definition currently looks like this:

def name(positional_or_keyword_parameters, *, keyword_only_parameters):

Building on that perspective, the new syntax for functions would look like this:

def name(positional_only_parameters, /, positional_or_keyword_parameters,
         *, keyword_only_parameters):

All parameters before the / are positional-only. If / is not specified in a function signature, that function does not accept any positional-only parameters. The logic around optional values for positional-only argument Remains the same as the one for positional-or-keyword. Once a positional-only argument is provided with a default, the following positional-only and positional-or-keyword argument need to have a default as well. Positional-only parameters that don’t have a default value are "required" positional-only parameters. Therefore the following are valid signatures:

def name(p1, p2, /, p_or_kw, *, kw):
def name(p1, p2=None, /, p_or_kw=None, *, kw):
def name(p1, p2=None, /, *, kw):
def name(p1, p2=None, /):
def name(p1, p2, /, p_or_kw):
def name(p1, p2, /):

Whilst the followings are not:

def name(p1, p2=None, /, p_or_kw, *, kw):
def name(p1=None, p2, /, p_or_kw=None, *, kw):
def name(p1=None, p2, /):

Full grammar specification

A draft of the proposed grammar specification is:

  tfpdef (',' tfpdef)* ',' '/' [',' [typedargslist]] | typedargslist

  vfpdef (',' vfpdef)* ',' '/' [',' [varargslist]] | varargslist

It will be added to the actual typedargslist and varargslist but for easier discussion is presented as new_typedargslist and new_varargslist

Implementation Plan

The implementation will involve a full change of the Grammar. This will involve following the steps outlined in PEP 306 [7]. In addition, other steps are needed including:

  • Modifying the code object and the function object to be aware of positional only arguments.
  • Modifiying ceval.c (PyEval_EvalCodeEx, PyEval_EvalFrameEx...) to correctly handle positional-only arguments.
  • Modifying marshal.c to account for the modifications of the code object.

This does not intend to be a guide or a comprehensive recipe on how to implement this but a rough outline of the changes this will make to the codebase.

The advantages of this implementation involve speed, consistency with the implementation of keyword-only parameters as in PEP 3102 and a simpler implementation of all the tools and modules that will be impacted by this change.

Rejected Ideas

Do Nothing

Always an option, just not adding it. It was considered though that the benefits of adding it is worth the complexity it adds to the language.

After marker proposal

A complaint against the proposal is the fact that the modifier of the signature impacts the "already passed" tokens.

This might make confusing to "human parsers" to read functions with many arguments. Example:

def really_bad_example_of_a_python_function(fist_long_argument, second_long_argument,
                                            third_long_argument, /):

It is not until you reach the end of the signature that the reader realized the / and therefore the fact that the arguments are position-only. This deviates from how the keyword-only marker works.

That said we could not find an implementation that would modify the arguments after the marker, as that will force the one before the marker to be position only as well. Example:

def (x, y, /, z):

If we define that / makes only z position-only it won't be possible to call x and y via keyword argument. Finding a way to work around it will add confusion given that at the moment keyword arguments cannot be followed by positional arguments. / will therefore make both the preceding and following position-only.

Per-argument marker

Using a per argument marker might be an option as well. The approach basically adds a token to each of the arguments that are position only and requires those to be placed together. Example:

def (.arg1, .arg2, arg3):

Note the dot on arg1 and arg2. Even if this approach might look easier to read it has been discarded as / goes further inline with the keyword-only approach and is less error prone.

Using decorators

It has been suggested on python-ideas [8] to provide a decorator written in Python as an implementation for this feature. This approach has the advantage that keeps parameter declaration more easy to read but also introduces an asymmetry on how parameter behaviour is declared. Also, as the / syntax is already introduced for C functions, this inconsistency will make more difficult to implement all tools and modules that deal with this syntax including but not limited to, the argument clinic, the inspect module and the ast module. Another disadvantage of this approach is that calling the decorated functions will be slower than the functions generated if the feature was implemented directly in C.