# HG changeset patch
# User Paul Boddie <paul@boddie.org.uk>
# Date 1530626522 -7200
# Node ID 8e119abbd91c310b0697e53ddc84600af4577cb9
# Parent  0cac8b718fdc44ddb75b42187fcc9f95e9da5ed1
Introduced a class (unpackable) and method (__get_single_item_unchecked__) to
support sequence unpacking without index checking overhead, since the translator
emits a test for the length of each sequence and controls the actual index
values used.

diff -r 0cac8b718fdc -r 8e119abbd91c common.py
--- a/common.py	Tue Jul 03 15:58:44 2018 +0200
+++ b/common.py	Tue Jul 03 16:02:02 2018 +0200
@@ -462,7 +462,7 @@
         for i, node in enumerate(n.nodes):
             statements.append(
                 compiler.ast.Assign([node], compiler.ast.CallFunc(
-                    compiler.ast.Getattr(compiler.ast.Name(temp), "__get_single_item__"),
+                    compiler.ast.Getattr(compiler.ast.Name(temp), "__get_single_item_unchecked__"),
                     [compiler.ast.Const(i, str(i))]))
                 )
 
diff -r 0cac8b718fdc -r 8e119abbd91c lib/__builtins__/list.py
--- a/lib/__builtins__/list.py	Tue Jul 03 15:58:44 2018 +0200
+++ b/lib/__builtins__/list.py	Tue Jul 03 16:02:02 2018 +0200
@@ -3,7 +3,7 @@
 """
 List objects.
 
-Copyright (C) 2015, 2016, 2017 Paul Boddie <paul@boddie.org.uk>
+Copyright (C) 2015, 2016, 2017, 2018 Paul Boddie <paul@boddie.org.uk>
 
 This program is free software; you can redistribute it and/or modify it under
 the terms of the GNU General Public License as published by the Free Software
@@ -20,11 +20,11 @@
 """
 
 from __builtins__.iteration.iterator import itemiterator
-from __builtins__.sequence import sequence, _get_absolute_index
+from __builtins__.sequence import unpackable, _get_absolute_index
 from native import list_append, list_concat, list_element, list_init, \
                    list_len, list_nonempty, list_setelement, list_setsize
 
-class list(sequence):
+class list(unpackable):
 
     "Implementation of list."
 
diff -r 0cac8b718fdc -r 8e119abbd91c lib/__builtins__/sequence.py
--- a/lib/__builtins__/sequence.py	Tue Jul 03 15:58:44 2018 +0200
+++ b/lib/__builtins__/sequence.py	Tue Jul 03 16:02:02 2018 +0200
@@ -20,7 +20,7 @@
 """
 
 from __builtins__.int import maxint
-from native import isinstance as _isinstance, is_int
+from native import isinstance as _isinstance, is_int, list_element
 
 class itemaccess:
 
@@ -255,6 +255,19 @@
 
     # __iter__(self)
 
+class unpackable(sequence):
+
+    "Class for list and tuple unpacking."
+
+    def __get_single_item_unchecked__(self, index):
+
+        """
+        NOTE: Should restrict this to internal translator use.
+        NOTE: This also uses implementation-specific access.
+        """
+
+        return list_element(self.__data__, index)
+
 def _get_absolute_index(index, length):
 
     """
diff -r 0cac8b718fdc -r 8e119abbd91c lib/__builtins__/tuple.py
--- a/lib/__builtins__/tuple.py	Tue Jul 03 15:58:44 2018 +0200
+++ b/lib/__builtins__/tuple.py	Tue Jul 03 16:02:02 2018 +0200
@@ -3,7 +3,7 @@
 """
 Tuple objects.
 
-Copyright (C) 2015, 2016, 2017 Paul Boddie <paul@boddie.org.uk>
+Copyright (C) 2015, 2016, 2017, 2018 Paul Boddie <paul@boddie.org.uk>
 
 This program is free software; you can redistribute it and/or modify it under
 the terms of the GNU General Public License as published by the Free Software
@@ -20,11 +20,11 @@
 """
 
 from __builtins__.iteration.iterator import itemiterator
-from __builtins__.sequence import hashable, sequence
+from __builtins__.sequence import hashable, unpackable
 from native import tuple_init, \
                    list_element, list_len, list_setsize, list_setelement
 
-class tuple(sequence, hashable):
+class tuple(unpackable, hashable):
 
     "Implementation of tuple."
 
@@ -65,7 +65,7 @@
         'step'.
         """
 
-        return tuple(get_using(sequence.__getslice__, self)(start, end, step))
+        return tuple(get_using(unpackable.__getslice__, self)(start, end, step))
 
     def __len__(self):