PEP: 457
Title: Notation For Positional-Only Parameters
Author: Larry Hastings <larry@hastings.org>
Discussions-To: python-dev@python.org
Status: Final
Type: Informational
Created: 08-Oct-2013


========
Overview
========

This PEP proposes a notation 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.

This PEP is an Informational PEP describing the notation for use when
describing APIs that use positional-only parameters (e.g. in Argument
Clinic, or in the string representation of ``inspect.Signature``
objects). A separate PEP, :pep:`570`, proposes elevation of this notation
to full Python syntax.

=========
Rationale
=========

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.
But, even in current versions of Python, many CPython
"builtin" functions still only accept positional-only
arguments.

Functions implemented in modern Python can accept
an arbitrary number of positional-only arguments, via the
variadic ``*args`` parameter.  However, there is no Python
syntax to specify accepting a specific number of
positional-only parameters.  Put another way, there are
many builtin functions whose signatures are simply not
expressible with Python syntax.

This PEP proposes a notation for such signatures that could form the
basis of a backwards-compatible syntax that should permit implementing
any builtin in pure Python code (see :pep:`570` for that proposal).

-----------------------------------------------------
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::

    >>> pow(x=5, y=3)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: pow() takes no keyword arguments

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

* ``range()``, which accepts an optional parameter
  to the *left* of its required parameter. [#RANGE]_

* ``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! [#DICT]_

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's signature and what it actually accepts,
not to mention the work of implementing said argument parsing.

==========
Motivation
==========

This PEP does not propose we implement positional-only
parameters in Python.  The goal of this PEP is simply
to define the syntax, so that:

* Documentation can clearly, unambiguously, and
  consistently express exactly how the arguments
  for a function will be interpreted.

* The syntax is reserved for future use, in case
  the community decides someday to add positional-only
  parameters to the language.

* Argument Clinic can use a variant of the syntax
  as part of its input when defining
  the arguments for built-in functions.

=================================================================
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. [#BORDER]_

* Some functions denote optional groups of positional-only arguments
  by presenting multiple prototypes with varying numbers of
  arguments. [#SENDFILE]_

* Some functions use *both* of the above approaches. [#RANGE]_ [#ADDCH]_

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.

Positional-only parameters can have a default value, and if they
do they are optional.  Positional-only parameters that don't have
a default value are "required" positional-only parameters.

More semantics of positional-only parameters:

* Although positional-only parameter technically have names,
  these names are internal-only; positional-only parameters
  are *never* externally addressable by name.  (Similarly
  to ``*args`` and ``**kwargs``.)

* If there are arguments after the ``/``, then you must specify
  a comma after the ``/``, just as there is a comma
  after the ``*`` denoting the shift to keyword-only parameters.

* This syntax has no effect on ``*args`` or ``**kwargs``.

======================
Additional Limitations
======================

Argument Clinic uses a form of this syntax for specifying
builtins.  It imposes further limitations that are
theoretically unnecessary but make the implementation
easier.  Specifically:

* A function that has positional-only parameters currently
  cannot have any other kind of parameter.  (This will
  probably be relaxed slightly in the near future.)

* Argument Clinic supports an additional syntax called
  "optional groups".  An "optional group" is a sequential
  set of positional-only parameters that must be specified
  or not-specified as a group.  If, for example, you define
  a function in Argument Clinic that takes four parameters,
  and all of them are positional-only and in one optional
  group, then when calling the function you must specify
  either zero arguments or four arguments.  This is necessary
  to cover more of Python's legacy library, but is outside
  the scope of this PEP, and is not recommended for actual
  inclusion in the Python language.

==============================
Notes For A Future Implementor
==============================

If we decide to implement positional-only parameters in a future
version of Python, we'd have to do some additional work to preserve
their semantics.  The problem: how do we inform a parameter that
no value was passed in for it when the function was called?

The obvious solution: add a new singleton constant to Python
that is passed in when a parameter is not mapped to an argument.
I propose that the value be called ``undefined``,
and be a singleton of a special class called ``Undefined``.
If a positional-only parameter did not receive an argument
when called, its value would be set to ``undefined``.

But this raises a further problem.  How do can we tell the
difference between "this positional-only parameter did not
receive an argument" and "the caller passed in ``undefined``
for this parameter"?

It'd be nice to make it illegal to pass ``undefined`` in
as an argument to a function--to, say, raise an exception.
But that would slow Python down, and the "consenting adults"
rule appears applicable here.  So making it illegal should
probably be strongly discouraged but not outright prevented.

However, it should be allowed (and encouraged) for user
functions to specify ``undefined`` as a default value for
parameters.

====================
Unresolved Questions
====================

There are three types of parameters in Python:

1. positional-only parameters,
2. positional-or-keyword parameters, and
3. keyword-only parameters.

Python allows functions to have both 2 and 3.  And some
builtins (e.g. range) have both 1 and 3.  Does it make
sense to have functions that have both 1 and 2?  Or
all of the above?


======
Thanks
======

Credit for the use of '/' as the separator between positional-only and positional-or-keyword
parameters goes to Guido van Rossum, in a proposal from 2012. [#GUIDO]_

Credit for making left option groups higher precedence goes to
Alyssa Coghlan. (Conversation in person at PyCon US 2013.)

.. [#DICT]
    http://docs.python.org/3/library/stdtypes.html#dict

.. [#RANGE]
    http://docs.python.org/3/library/functions.html#func-range

.. [#BORDER]
    http://docs.python.org/3/library/curses.html#curses.window.border

.. [#SENDFILE]
    http://docs.python.org/3/library/os.html#os.sendfile

.. [#ADDCH]
    http://docs.python.org/3/library/curses.html#curses.window.addch

.. [#GUIDO]
   Guido van Rossum, posting to python-ideas, March 2012:
   https://mail.python.org/pipermail/python-ideas/2012-March/014364.html
   and
   https://mail.python.org/pipermail/python-ideas/2012-March/014378.html
   and
   https://mail.python.org/pipermail/python-ideas/2012-March/014417.html

=========
Copyright
=========

This document has been placed in the public domain.
