python-peps/peps/pep-3132.rst

PEP: 3132
Title: Extended Iterable Unpacking
Version: $Revision$
Last-Modified: $Date$
Author: Georg Brandl <georg@python.org>
Status: Final
Type: Standards Track
Content-Type: text/x-rst
Created: 30-Apr-2007
Python-Version: 3.0
Post-History:


Abstract
========

This PEP proposes a change to iterable unpacking syntax, allowing to
specify a "catch-all" name which will be assigned a list of all items
not assigned to a "regular" name.

An example says more than a thousand words::

    >>> a, *b, c = range(5)
    >>> a
    0
    >>> c
    4
    >>> b
    [1, 2, 3]


Rationale
=========

Many algorithms require splitting a sequence in a "first, rest" pair.
With the new syntax, ::

    first, rest = seq[0], seq[1:]

is replaced by the cleaner and probably more efficient::

    first, *rest = seq

For more complex unpacking patterns, the new syntax looks even
cleaner, and the clumsy index handling is not necessary anymore.

Also, if the right-hand value is not a list, but an iterable, it
has to be converted to a list before being able to do slicing; to
avoid creating this temporary list, one has to resort to ::

    it = iter(seq)
    first = it.next()
    rest = list(it)


Specification
=============

A tuple (or list) on the left side of a simple assignment (unpacking
is not defined for augmented assignment) may contain at most one
expression prepended with a single asterisk (which is henceforth
called a "starred" expression, while the other expressions in the
list are called "mandatory").  This designates a subexpression that
will be assigned a list of all items from the iterable being unpacked
that are not assigned to any of the mandatory expressions, or an
empty list if there are no such items.

For example, if ``seq`` is a sliceable sequence, all the following
assignments are equivalent if ``seq`` has at least two elements::

    a, b, c = seq[0], list(seq[1:-1]), seq[-1]
    a, *b, c = seq
    [a, *b, c] = seq

It is an error (as it is currently) if the iterable doesn't contain
enough items to assign to all the mandatory expressions.

It is also an error to use the starred expression as a lone
assignment target, as in ::

    *a = range(5)

This, however, is valid syntax::

    *a, = range(5)

Note that this proposal also applies to tuples in implicit assignment
context, such as in a ``for`` statement::

    for a, *b in [(1, 2, 3), (4, 5, 6, 7)]:
        print(b)

would print out ::

    [2, 3]
    [5, 6, 7]

Starred expressions are only allowed as assignment targets, using them
anywhere else (except for star-args in function calls, of course) is an
error.


Implementation
==============

Grammar change
--------------

This feature requires a new grammar rule::

    star_expr: ['*'] expr

In these two rules, ``expr`` is changed to ``star_expr``::

    comparison: star_expr (comp_op star_expr)*
    exprlist: star_expr (',' star_expr)* [',']

Changes to the Compiler
-----------------------

A new ASDL expression type ``Starred`` is added which represents a
starred expression.  Note that the starred expression element
introduced here is universal and could later be used for other
purposes in non-assignment context, such as the ``yield *iterable``
proposal.

The compiler is changed to recognize all cases where a starred
expression is invalid and flag them with syntax errors.

A new bytecode instruction, ``UNPACK_EX``, is added, whose argument
has the number of mandatory targets before the starred target in the
lower 8 bits and the number of mandatory targets after the starred
target in the upper 8 bits.  For unpacking sequences without starred
expressions, the old ``UNPACK_ITERABLE`` opcode is kept.

Changes to the Bytecode Interpreter
-----------------------------------

The function ``unpack_iterable()`` in ceval.c is changed to handle
the extended unpacking, via an ``argcntafter`` parameter. In the
``UNPACK_EX`` case, the function will do the following:

* collect all items for mandatory targets before the starred one
* collect all remaining items from the iterable in a list
* pop items for mandatory targets after the starred one from the list
* push the single items and the resized list on the stack

Shortcuts for unpacking iterables of known types, such as lists or
tuples, can be added.


The current implementation can be found at the SourceForge Patch
tracker [SFPATCH]_. It now includes a minimal test case.


Acceptance
==========

After a short discussion on the python-3000 list [1]_, the PEP was
accepted by Guido in its current form.  Possible changes discussed
were:

* Only allow a starred expression as the last item in the exprlist.
  This would simplify the unpacking code a bit and allow for the
  starred expression to be assigned an iterator.  This behavior was
  rejected because it would be too surprising.

* Try to give the starred target the same type as the source
  iterable, for example, ``b`` in ``a, *b = 'hello'`` would be
  assigned the string ``'ello'``.  This may seem nice, but is
  impossible to get right consistently with all iterables.

* Make the starred target a tuple instead of a list.  This would be
  consistent with a function's ``*args``, but make further processing
  of the result harder.


References
==========

.. [SFPATCH] https://bugs.python.org/issue1711529
.. [1] https://mail.python.org/pipermail/python-3000/2007-May/007198.html


Copyright
=========

This document has been placed in the public domain.



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