Georg Brandl wrote:
Nick Coghlan schrieb:
2. Method lookup MAY bypass __getattribute__, shadowing the attribute
in the instance dictionary MAY have ill effects. (slots such as
__enter__ and __exit__ that are looked up via normal attribute lookup
in CPython will fit into this category)
I would argue that the __enter__ and __exit__ behavior should be changed
too.
The reason for the current behavior is this:
2 0 LOAD_GLOBAL 0 (a)
3 DUP_TOP
4 LOAD_ATTR 1 (__exit__)
7 STORE_FAST 0 (_[1])
10 LOAD_ATTR 2 (__enter__)
13 CALL_FUNCTION 0
16 STORE_FAST 1 (_[2])
19 SETUP_FINALLY 18 (to 40)
IOW, when "with" is compiled, the attributes are retrieved using bytecode.
It wouldn't be hard to have a WITH_SETUP opcode (along with the already
existing WITH_CLEANUP) that would make the bytecode read like:
2 0 LOAD_GLOBAL 0 (a)
3 WITH_SETUP 0 (_[1])
6 STORE_FAST 1 (_[2])
9 SETUP_FINALLY 18 (to 40)
No argument here - the PEP 343 implementation is the way it is mainly
because it involved the least messing around with the
performance-sensitive ceval loop.
That said, the with statement implementation is already a bit different
in 2.6/3.0 (it moves things around on the stack so it can avoid the
STORE_FAST/LOAD_FAST/DELETE_FAST operations):
def f():
with a:
pass
Python 2.5 disassembly:
2 0 LOAD_GLOBAL 0 (a)
3 DUP_TOP
4 LOAD_ATTR 1 (__exit__)
7 STORE_FAST 0 (_[1])
10 LOAD_ATTR 2 (__enter__)
13 CALL_FUNCTION 0
16 POP_TOP
17 SETUP_FINALLY 4 (to 24)
20 POP_BLOCK
21 LOAD_CONST 0 (None)
>> 24 LOAD_FAST 0 (_[1])
27 DELETE_FAST 0 (_[1])
30 WITH_CLEANUP
31 END_FINALLY
Python 2.6 disassembly:
2 0 LOAD_GLOBAL 0 (a)
3 DUP_TOP
4 LOAD_ATTR 1 (__exit__)
7 ROT_TWO
8 LOAD_ATTR 2 (__enter__)
11 CALL_FUNCTION 0
14 POP_TOP
15 SETUP_FINALLY 4 (to 22)
18 POP_BLOCK
19 LOAD_CONST 0 (None)
>> 22 WITH_CLEANUP
23 END_FINALLY
Cheers,
Nick.
--
Nick Coghlan | [EMAIL PROTECTED] | Brisbane, Australia
---------------------------------------------------------------
http://www.boredomandlaziness.org
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