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PEP:448
Title:Additional Unpacking Generalizations
Version:bf59e2ae3b3c
Last-Modified:2015-02-26 12:17:47 -0800 (Thu, 26 Feb 2015)
Author:Joshua Landau <joshua at landau.ws>
Discussions-To:python-ideas at python.org
Status:Accepted
Type:Standards Track
Content-Type:text/x-rst
Created:29-Jun-2013
Python-Version:3.5
Post-History:

Abstract

This PEP proposes extended usages of the * iterable unpacking operator and ** dictionary unpacking operators to allow unpacking in more positions, an arbitrary number of times, and in additional circumstances. Specifically, in function calls, in comprehensions and generator expressions, and in displays.

Function calls are proposed to support an arbitrary number of unpackings rather than just one:

>>> print(*[1], *[2], 3)
1 2 3
>>> dict(**{'x': 1}, y=2, **{'z': 3})
{'x': 1, 'y': 2, 'z': 3}

Unpacking is proposed to be allowed inside tuple, list, set, and dictionary displays:

>>> *range(4), 4
(0, 1, 2, 3, 4)
>>> [*range(4), 4]
[0, 1, 2, 3, 4]
>>> {*range(4), 4}
{0, 1, 2, 3, 4}
>>> {'x': 1, **{'y': 2}}
{'x': 1, 'y': 2}

In dictionaries, later values will always override earlier ones:

>>> {'x': 1, **{'x': 2}}
{'x': 2}

>>> {**{'x': 2}, 'x': 1}
{'x': 1}

This PEP does not include unpacking operators inside list, set and dictionary comprehensions although this has not been ruled out for future proposals.

Rationale

Current usage of the * iterable unpacking operator features unnecessary restrictions that can harm readability.

Unpacking multiple times has an obvious rationale. When you want to unpack several iterables into a function definition or follow an unpack with more positional arguments, the most natural way would be to write:

function(**kw_arguments, **more_arguments)

function(*arguments, argument)

Simple examples where this is useful are print and str.format. Instead, you could be forced to write:

kwargs = dict(kw_arguments)
kwargs.update(more_arguments)
function(**kwargs)

args = list(arguments)
args.append(arg)
function(*args)

or, if you know to do so:

from collections import ChainMap
function(**ChainMap(more_arguments, arguments))

from itertools import chain
function(*chain(args, [arg]))

which add unnecessary line-noise and, with the first methods, causes duplication of work.

There are two primary rationales for unpacking inside of containers. Firstly there is a symmetry of assignment, where fst, *other, lst = elems and elems = fst, *other, lst are approximate inverses, ignoring the specifics of types. This, in effect, simplifies the language by removing special cases.

Secondly, it vastly simplifies types of "addition" such as combining dictionaries, and does so in an unambiguous and well-defined way:

combination = {**first_dictionary, "x": 1, "y": 2}

instead of:

combination = first_dictionary.copy()
combination.update({"x": 1, "y": 2})

which is especially important in contexts where expressions are preferred. This is also useful as a more readable way of summing iterables into a list, such as my_list + list(my_tuple) + list(my_range) which is now equivalent to just [*my_list, *my_tuple, *my_range].

Specification

Function calls may accept an unbounded number of * and ** unpackings. There will be no restriction of the order of positional arguments with relation to * unpackings nor any restriction of the order of keyword arguments with relation to ** unpackings.

Function calls continue to have the restriction that keyword arguments must follow positional arguments and ** unpackings must additionally follow * unpackings.

Currently, if an argument is given multiple times — such as a positional argument given both positionally and by keyword — a TypeError is raised. This remains true for duplicate arguments provided through multiple ** unpackings, e.g. f(**{'x': 2}, **{'x': 3}), except that the error will be detected at runtime.

A function looks like this:

function(
    argument or *args, argument or *args, ...,
    kwargument or *args, kwargument or *args, ...,
    kwargument or **kwargs, kwargument or **kwargs, ...
)

Tuples, lists, sets and dictionaries will allow unpacking. This will act as if the elements from unpacked items were inserted in order at the site of unpacking, much as happens in unpacking in a function-call. Dictionaries require ** unpacking; all the others require * unpacking.

The keys in a dictionary remain in a right-to-left priority order, so {**{'a': 1}, 'a': 2, **{'a': 3}} evaluates to {'a': 3}. There is no restriction on the number or position of unpackings.

Disadvantages

The allowable orders for arguments in a function call are more complicated than before. The simplest explanation for the rules may be "positional arguments precede keyword arguments and ** unpacking; * unpacking precedes ** unpacking".

Whilst *elements, = iterable causes elements to be a list, elements = *iterable, causes elements to be a tuple. The reason for this may confuse people unfamiliar with the construct.

Concerns have been raised about the unexpected difference between duplicate keys in dictionaries being allowed but duplicate keys in function call syntax raising an error. Although this is already the case with current syntax, this proposal might exacerbate the issue. It remains to be seen how much of an issue this is in practice.

Variations

The PEP originally considered whether the ordering of argument types in a function call (positional, keyword, * or **) could become less strict. This met little support so the idea was shelved.

Earlier iterations of this PEP allowed unpacking operators inside list, set, and dictionary comprehensions as a flattening operator over iterables of containers:

>>> ranges = [range(i) for i in range(5)]
>>> [*item for item in ranges]
[0, 0, 1, 0, 1, 2, 0, 1, 2, 3]

>>> {*item for item in ranges}
{0, 1, 2, 3}

This was met with a mix of strong concerns about readability and mild support. In order not to disadvantage the less controversial aspects of the PEP, this was not accepted with the rest of the proposal.

Unbracketed comprehensions in function calls, such as f(x for x in it), are already valid. These could be extended to:

f(*x for x in it) == f((*x for x in it))
f(**x for x in it) == f({**x for x in it})

However, it wasn't clear if this was the best behaviour or if it should unpack into the arguments of the call to f. Since this is likely to be confusing and is of only very marginal utility, it is not included in this PEP. Instead, these will throw a SyntaxError and comprehensions with explicit brackets should be used instead.

Approval

This PEP was accepted by Guido on February 25, 2015 [1].

Implementation

An implementation for Python 3.5 is found at Issue 2292 on bug tracker [2]. This currently includes support for unpacking inside comprehensions, which should be removed.

References

[1]PEP accepted, "PEP 448 review", Guido van Rossum (https://mail.python.org/pipermail/python-dev/2015-February/138564.html)
[2]Issue 2292, "Missing *-unpacking generalizations", Thomas Wouters (http://bugs.python.org/issue2292)
[3]Discussion on Python-ideas list, "list / array comprehensions extension", Alexander Heger (http://mail.python.org/pipermail/python-ideas/2011-December/013097.html)