335 lines
12 KiB
Plaintext
335 lines
12 KiB
Plaintext
PEP: 457
|
||
Title: Syntax For Positional-Only Parameters
|
||
Version: $Revision$
|
||
Last-Modified: $Date$
|
||
Author: Larry Hastings <larry@hastings.org>
|
||
Discussions-To: Python-Dev <python-dev@python.org>
|
||
Status: Draft
|
||
Type: Informational
|
||
Content-Type: text/x-rst
|
||
Created: 08-Oct-2013
|
||
|
||
|
||
========
|
||
Overview
|
||
========
|
||
|
||
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.
|
||
|
||
=========
|
||
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
|
||
expressable with Python syntax.
|
||
|
||
This PEP proposes a backwards-compatible syntax that should
|
||
permit implementing any builtin in pure Python code.
|
||
|
||
-----------------------------------------------------
|
||
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 be optional, but the mechanism is
|
||
significantly different from positional-or-keyword or keyword-only
|
||
parameters. Positional-only parameters don't accept default
|
||
values. Instead, positional-only parameters can be specified
|
||
in optional "groups". Groups of parameters are surrounded by
|
||
square brackets, like so::
|
||
|
||
def addch([y, x,] ch, [attr,] /):
|
||
|
||
Positional-only parameters that are not in an option group are
|
||
"required" positional-only parameters. All "required" positional-only
|
||
parameters must be contiguous.
|
||
|
||
Parameters in an optional group accept arguments in a group; you
|
||
must provide arguments either for all of the them or for none of them.
|
||
Using the example of ``addch()`` above, you could not call ``addch()``
|
||
in such a way that ``x`` was specified but ``y`` was not (and vice versa).
|
||
The mapping of positional parameters to optional groups is done
|
||
based on fitting the number of parameters to groups. Based on the
|
||
above definition, ``addch()`` would assign arguments to parameters
|
||
in the following way:
|
||
|
||
+-------------------+------------------------------+
|
||
|Number of arguments|Parameter assignment |
|
||
+-------------------+------------------------------+
|
||
|0 |*raises an exception* |
|
||
+-------------------+------------------------------+
|
||
|1 |``ch`` |
|
||
+-------------------+------------------------------+
|
||
|2 |``ch``, ``attr`` |
|
||
+-------------------+------------------------------+
|
||
|3 |``y``, ``x``, ``ch`` |
|
||
+-------------------+------------------------------+
|
||
|4 |``y``, ``x``, ``ch``, ``attr``|
|
||
+-------------------+------------------------------+
|
||
|5 or more |*raises an exception* |
|
||
+-------------------+------------------------------+
|
||
|
||
|
||
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``.)
|
||
|
||
* It's possible to nest option groups.
|
||
|
||
* If there are no required parameters, all option groups behave
|
||
as if they're to the right of the required parameter group.
|
||
|
||
* For clarity and consistency, the comma for a parameter always
|
||
comes immediately after the parameter name. It's a syntax error
|
||
to specify a square bracket between the name of a parameter and
|
||
the following comma. (This is far more readable than putting
|
||
the comma outside the square bracket, particularly for nested
|
||
groups.)
|
||
|
||
* 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``.
|
||
|
||
It's possible to specify a function prototype where the mapping
|
||
of arguments to parameters is ambiguous. Consider::
|
||
|
||
def range([start,] stop, [range,] /):
|
||
|
||
Python disambiguates these situations by preferring optional groups
|
||
to the *left* of the required group.
|
||
|
||
======================
|
||
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.)
|
||
|
||
* Multiple option groups on either side of the required
|
||
positional-only parameters must be nested, with the
|
||
nesting getting deeper the further away the group is
|
||
from the required positional-parameter group.
|
||
|
||
Put another way:
|
||
all the left-brackets for option groups to the
|
||
left of the required group must be specified contiguously,
|
||
and
|
||
all the right-brackets for option groups to the
|
||
right of the required group must be specified contiguously.
|
||
|
||
|
||
==============================
|
||
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
|
||
Nick 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:
|
||
http://mail.python.org/pipermail/python-ideas/2012-March/014364.html
|
||
and
|
||
http://mail.python.org/pipermail/python-ideas/2012-March/014378.html
|
||
and
|
||
http://mail.python.org/pipermail/python-ideas/2012-March/014417.html
|
||
|
||
=========
|
||
Copyright
|
||
=========
|
||
|
||
This document has been placed in the public domain.
|
||
|
||
|
||
|
||
..
|
||
Local Variables:
|
||
mode: indented-text
|
||
indent-tabs-mode: nil
|
||
sentence-end-double-space: t
|
||
fill-column: 70
|
||
coding: utf-8
|
||
End:
|