PEP 3102 for review and comment

[Talin]

(Note: PEPs in the 3xxx number range are intended for Python 3000,
however this particular PEP may be backported if there is support for
it.)

PEP: 3102
Title: Keyword-Only Arguments
Version: $Revision: 46053 $
Last-Modified: $Date: 2006-05-19 22:23:44 -0700 (Fri, 19 May 2006) $
Author: Talin <talin at acm.org>
Status: Draft
Type: Standards
Content-Type: text/plain
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:

o 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'.

o If the next unfilled slot is a vararg slot, and it does
not have a name, then it is an error.

o 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:

o 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.

o 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.

o Otherwise, if there is no keyword dictionary, and no
matching named parameter, then it is an error.

- Finally:

o If the vararg slot is not yet filled, assign an empty tuple
as its value.

o 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.


Copyright

This document has been placed in the public domain.


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End:

 

[Carl Banks]

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.

Ick. Pretty big change hinging on subtle juxtaposition in the argument
list. I don't like it, it doesn't stand out enough... but it makes
logical sense and any other solution would have to be way more
contrived. I think argument lists are already confusing enough with
just mixing *, **, and default arguments.

Two thoughts:

Would these arguments go before of after **kwargs?

This is apparently for Python 3.0. (I'd make the implementation
schedule explicit, even if the PEP number implies it's Python 3.0.) It
looks like Python 3.0 might declare function arguments like Ada. If
so, what about a little modifier (like Ada's "in and out") instead?
Problem with that is then positional arguments line up wrong. Ick.

def func(a : int = 1): pass
def func(a : kwonly int = 1): pass


Carl Banks

 

[Russell E. Owen]

(Note: PEPs in the 3xxx number range are intended for Python 3000,
however this particular PEP may be backported if there is support for
it.)

PEP: 3102
Title: Keyword-Only Arguments ....
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.

The following is a 2nd syntactical change, and I strongly suggest
listing it as such (since the first change can be made without this
change):

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.

....making this the third syntactical change:

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):

Personally, my feelings are:

+1 on allowing keyword arguments after *args. I have long wanted this
and feel it is:
- very useful (the alternative of abusing **kargs is ugly and doesn't
support introspection)
- an obvious and clear extension of current notation
- removes an awkward limitation of current notation

-1 on allowing keywords without values because:
- if it is added, then it means something like the last change is
required, and as I say next, I don't like it
- why bother? it just makes work for the caller

-1 on the use of * to mean a separator between positional args and
keyword-only args because:
- I think proposed syntax is confusing (there are more arguments in the
argument list than the function accepts; yecch!!!) and hard to read
- I don't think the added functionality is important enough to justify
the awkward notation

If folks are desperate enough for this last feature then please at least
find a clearer notation. For example:
def compare(a, b | key=None):
but personally I'd rather skip it.

Now if somebody could figure out a natural notation for boolean flags,
*that'd* be useful (where the presence, absence or explicit negation of
a keyword is all that is required to enable or disable a feature).

-- Russell

 

[Talin]

Allowing keyword arguments without defaults may seem like a syntactical
change, but in fact it's not. You can do this in Python now - any
argument can be a 'keyword' argument, whether it has a default value or
not. So for example:

def func( a, b, c=0 ):
pass

func( a=1, b=2, c=3 )

In other words, the use of '=' to indicate a keyword argument, and the
use of '=' to indicate a default argument value have *nothing* to do
with each other, other than the fact that they both have the
requirement that they have to come after positional arguments.

If you wanted to require keyword-only arguments to have default values,
that would require adding an additional check, which would be a
syntactical change.

-- Talin