PEP: 3132 Title: Extended Iterable Unpacking Version: $Revision$ Last-Modified: $Date$ Author: Georg Brandl Status: Draft 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. 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. For the rest of this section, the other expressions in the list are called "mandatory". Note that this also refers to tuples in implicit assignment context, such as in a ``for`` statement. 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. It is an error (as it is currently) if the iterable doesn't contain enough items to assign to all the mandatory expressions. Implementation ============== The proposed implementation strategy is: - add a new grammar rule, ``star_test``, which consists of ``'*' test`` and is used in test lists - add a new ASDL type ``Starred`` to represent a starred expression - catch all cases where starred expressions are not allowed in the AST and symtable generation stage - add a new opcode, ``UNPACK_EX``, which will only be used if a list/tuple to be assigned to contains a starred expression - change ``unpack_iterable()`` in ceval.c to handle the extended unpacking case Note that the starred expression element introduced here is universal and could be used for other purposes in non-assignment context, such as the ``yield *iterable`` proposal. The author has written a draft implementation, but there are some open issues which will be resolved in case this PEP is looked upon benevolently. Open Issues =========== - Should the catch-all expression be assigned a list or a tuple of items? References ========== None yet. Copyright ========= This document has been placed in the public domain. .. Local Variables: mode: indented-text indent-tabs-mode: nil sentence-end-double-space: t fill-column: 70 coding: utf-8 End: