|Title:||Notation For Positional-Only Parameters|
|Author:||Larry Hastings <larry at hastings.org>|
|Discussions-To:||Python-Dev <python-dev at python.org>|
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.
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).
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. 
- 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! 
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.
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 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. 
- Some functions denote optional groups of positional-only arguments by presenting multiple prototypes with varying numbers of arguments. 
- Some functions use both of the above approaches.  
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.
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.
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.
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.
There are three types of parameters in Python:
- positional-only parameters,
- positional-or-keyword parameters, and
- 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?
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. 
Credit for making left option groups higher precedence goes to Nick Coghlan. (Conversation in person at PyCon US 2013.)
|||(1, 2) http://docs.python.org/3/library/functions.html#func-range|
|||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|
This document has been placed in the public domain.