[issue12635] use "as" for block scope support

[HaiYun Yan]

New submission from HaiYun Yan <lyricconch at gmail.com>:

use "as" for block scope support
just like mozilla javascript "let" extension
https://developer.mozilla.org/en/new_in_javascript_1.7#Block_scope_with_let_%28Merge_into_let_Statement%29

usage:

as_clause(as_declare): "as" NAME
as_stmt: NAME "as" test
as_expr: "(" expr "as" NAME ")" expr

var declare by as_clause is alive in <suite> only
eg:
<NAME "f" binding saved>with open(...) as f<NAME "f" bind to HEAPObj>:
   ...
<NAME "f" binding restored>

the same rule to "except E as e: "

spec:
from socket import AF_INET as IPv4
<IPv4 bind to socket.AF_INET slot>
IPv4 = None
assert socket.AF_INET == None

var in as_stmt is alive in current indentation
eg:
for i in range(10):
   <NAME "t" binding saved>
   t as f()+1 <NAME "t" bind to f()+1>
   ...
   <NAME "t" binding restored>

var in as_expr is alive in sub expr only
eg:
t = None
x = (2+3 as t) t**2
assert t == None ane  x == 25

Suggestion:
new opcode
ENTER_BLOCK #depth #varslots

----------
components: Interpreter Core
messages: 141076
nosy: lyricconch
priority: normal
severity: normal
status: open
title: use "as" for block scope support
type: feature request

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue12635>
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