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PEP:457
Title:Syntax For Positional-Only Parameters
Version:1b5ccbf217af
Last-Modified:2013-10-09 02:56:36 +0200 (Wed, 09 Oct 2013)
Author:Larry Hastings <larry at hastings.org>
Discussions-To:Python-Dev <python-dev at 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. [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'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. [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.

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. [6]

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

[1]http://docs.python.org/3/library/stdtypes.html#dict
[2](1, 2) http://docs.python.org/3/library/functions.html#func-range
[3]http://docs.python.org/3/library/curses.html#curses.window.border
[4]http://docs.python.org/3/library/os.html#os.sendfile
[5]http://docs.python.org/3/library/curses.html#curses.window.addch
[6]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