[Python-checkins] r55084 - peps/trunk/pep-3132.txt

[georg.brandl]

Author: georg.brandl
Date: Wed May  2 23:24:11 2007
New Revision: 55084

Modified:
   peps/trunk/pep-3132.txt
Log:
Update PEP and refer to patch.


Modified: peps/trunk/pep-3132.txt
==============================================================================
--- peps/trunk/pep-3132.txt	(original)
+++ peps/trunk/pep-3132.txt	Wed May  2 23:24:11 2007
@@ -44,49 +44,109 @@
 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.  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.
+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 slicable sequence, all the following
+assignments are equivalent if ``seq`` has at least three elements::
+
+    a, b, c = seq[0], 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]
+
 
 Implementation
 ==============
 
-The proposed implementation strategy is:
+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.
+
 
-- 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.
+The current implementation can be found at the SourceForge Patch
+tracker [SFPATCH]_. It now includes a minimal test case.
 
 
 Open Issues
@@ -98,7 +158,7 @@
 References
 ==========
 
-None yet.
+.. [SFPATCH] http://python.org/sf/1711529
 
 
 Copyright