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.