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PEP 3102 -- Keyword-Only Arguments

PEP:3102
Title:Keyword-Only Arguments
Author:Talin <viridia at gmail.com>
Status:Final
Type:Standards Track
Created:22-Apr-2006
Python-Version:3.0
Post-History:28-Apr-2006, May-19-2006

Abstract

This PEP proposes a change to the way that function arguments are assigned to named parameter slots. In particular, it enables the declaration of "keyword-only" arguments: arguments that can only be supplied by keyword and which will never be automatically filled in by a positional argument.

Rationale

The current Python function-calling paradigm allows arguments to be specified either by position or by keyword. An argument can be filled in either explicitly by name, or implicitly by position.

There are often cases where it is desirable for a function to take a variable number of arguments. The Python language supports this using the 'varargs' syntax (*name), which specifies that any 'left over' arguments be passed into the varargs parameter as a tuple.

One limitation on this is that currently, all of the regular argument slots must be filled before the vararg slot can be.

This is not always desirable. One can easily envision a function which takes a variable number of arguments, but also takes one or more 'options' in the form of keyword arguments. Currently, the only way to do this is to define both a varargs argument, and a 'keywords' argument (**kwargs), and then manually extract the desired keywords from the dictionary.

Specification

Syntactically, the proposed changes are fairly simple. The first change is to allow regular arguments to appear after a varargs argument:

def sortwords(*wordlist, case_sensitive=False):
    ...

This function accepts any number of positional arguments, and it also accepts a keyword option called 'case_sensitive'. This option will never be filled in by a positional argument, but must be explicitly specified by name.

Keyword-only arguments are not required to have a default value. Since Python requires that all arguments be bound to a value, and since the only way to bind a value to a keyword-only argument is via keyword, such arguments are therefore 'required keyword' arguments. Such arguments must be supplied by the caller, and they must be supplied via keyword.

The second syntactical change is to allow the argument name to be omitted for a varargs argument. The meaning of this is to allow for keyword-only arguments for functions that would not otherwise take a varargs argument:

def compare(a, b, *, key=None):
    ...

The reasoning behind this change is as follows. Imagine for a moment a function which takes several positional arguments, as well as a keyword argument:

def compare(a, b, key=None):
    ...

Now, suppose you wanted to have 'key' be a keyword-only argument. Under the above syntax, you could accomplish this by adding a varargs argument immediately before the keyword argument:

def compare(a, b, *ignore, key=None):
    ...

Unfortunately, the 'ignore' argument will also suck up any erroneous positional arguments that may have been supplied by the caller. Given that we'd prefer any unwanted arguments to raise an error, we could do this:

def compare(a, b, *ignore, key=None):
    if ignore:  # If ignore is not empty
        raise TypeError

As a convenient shortcut, we can simply omit the 'ignore' name, meaning 'don't allow any positional arguments beyond this point'.

(Note: After much discussion of alternative syntax proposals, the BDFL has pronounced in favor of this 'single star' syntax for indicating the end of positional parameters.)

Function Calling Behavior

The previous section describes the difference between the old behavior and the new. However, it is also useful to have a description of the new behavior that stands by itself, without reference to the previous model. So this next section will attempt to provide such a description.

When a function is called, the input arguments are assigned to formal parameters as follows:

  • For each formal parameter, there is a slot which will be used to contain the value of the argument assigned to that parameter.
  • Slots which have had values assigned to them are marked as 'filled'. Slots which have no value assigned to them yet are considered 'empty'.
  • Initially, all slots are marked as empty.
  • Positional arguments are assigned first, followed by keyword arguments.
  • For each positional argument:
    • Attempt to bind the argument to the first unfilled parameter slot. If the slot is not a vararg slot, then mark the slot as 'filled'.
    • If the next unfilled slot is a vararg slot, and it does not have a name, then it is an error.
    • Otherwise, if the next unfilled slot is a vararg slot then all remaining non-keyword arguments are placed into the vararg slot.
  • For each keyword argument:
    • If there is a parameter with the same name as the keyword, then the argument value is assigned to that parameter slot. However, if the parameter slot is already filled, then that is an error.
    • Otherwise, if there is a 'keyword dictionary' argument, the argument is added to the dictionary using the keyword name as the dictionary key, unless there is already an entry with that key, in which case it is an error.
    • Otherwise, if there is no keyword dictionary, and no matching named parameter, then it is an error.
  • Finally:
    • If the vararg slot is not yet filled, assign an empty tuple as its value.
    • For each remaining empty slot: if there is a default value for that slot, then fill the slot with the default value. If there is no default value, then it is an error.

In accordance with the current Python implementation, any errors encountered will be signaled by raising TypeError. (If you want something different, that's a subject for a different PEP.)

Backwards Compatibility

The function calling behavior specified in this PEP is a superset of the existing behavior - that is, it is expected that any existing programs will continue to work.

Source: https://github.com/python/peps/blob/master/pep-3102.txt