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Currently, 'with' costs about .2us over try/finally:
$ ./python.exe -m timeit -s 'import thread; lock =
thread.allocate_lock()' 'lock.acquire()' 'try: pass' 'finally:
lock.release()'
1000000 loops, best of 3: 0.617 usec per loop
$ ./python.exe -m timeit -s 'import thread; lock =
thread.allocate_lock()' 'with lock: pass'
1000000 loops, best of 3: 0.774 usec per loop
Since it's doing the same thing (and calling the same C functions to do
the lock manipulation), it shouldn't take more time. The bytecodes
associated with the two options look like:
2) with lock:
3) pass
 2 0 LOAD_GLOBAL 0 (lock)
 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)
 3 20 POP_BLOCK 
 21 LOAD_CONST 0 (None)
 >> 24 LOAD_FAST 0 (_[1])
 27 DELETE_FAST 0 (_[1])
 30 WITH_CLEANUP 
 31 END_FINALLY 
 32 LOAD_CONST 0 (None)
 35 RETURN_VALUE 
6) lock.acquire()
7) try:
8) pass
9) finally:
10) lock.release()
 6 0 LOAD_GLOBAL 0 (lock)
 3 LOAD_ATTR 1 (acquire)
 6 CALL_FUNCTION 0
 9 POP_TOP 
 7 10 SETUP_FINALLY 4 (to 17)
 8 13 POP_BLOCK 
 14 LOAD_CONST 0 (None)
 10 >> 17 LOAD_GLOBAL 0 (lock)
 20 LOAD_ATTR 2 (release)
 23 CALL_FUNCTION 0
 26 POP_TOP 
 27 END_FINALLY 
 28 LOAD_CONST 0 (None)
 31 RETURN_VALUE
The major difference I see is the extra local variable (_[1]) used by
with. It looks like the compiler ought to be able to save that on the
stack instead, and save 3 opcodes. Neal Norwitz suggested that, if that
optimization is impossible, WITH_CLEANUP could be modified to take the
variable as a parameter, which would let it absorb the LOAD_FAST and
DELETE_FAST instructions.
I've added everyone on the previous bug to the nosy list. Sorry if you
don't care. :)

A closer approximation of what the with statement is doing would be:
exit = lock.release()
lock.acquire()
try:
 pass
finally:
 exit()
The problem with trying to store the result of the retrieval of __exit__
on the stack is that we need to leave the context manager itself on top
of the stack for the next LOAD_ATTR opcode (when we're retrieving the
__enter__ method).
However, changing WITH_CLEANUP to take an argument indicating which
local variable holds the bound __exit__ method sounds like it might work.

Scratch the parentheses on that first line of sample code in my previous
comment (isn't copy and paste wonderful?)

Here's a proof-of-concept patch that keeps the __exit__ method on the
stack. It uses ROT_TWO to stuff it under the context object instead of
storing it into a temporary. (Thanks Nick for pointing out that problem
before I had to waste time on it.) test_with passes, although I need to
update several more things and maybe fix a refleak.
The patch changes the compilation of:
 def with_(l):
 with l:
 pass
from
 4 0 LOAD_FAST 0 (l)
 3 DUP_TOP 
 4 LOAD_ATTR 0 (__exit__)
 7 STORE_FAST 1 (_[1])
 10 LOAD_ATTR 1 (__enter__)
 13 CALL_FUNCTION 0
 16 POP_TOP 
 17 SETUP_FINALLY 4 (to 24)
 5 20 POP_BLOCK 
 21 LOAD_CONST 0 (None)
 >> 24 LOAD_FAST 1 (_[1])
 27 DELETE_FAST 1 (_[1])
 30 WITH_CLEANUP 
 31 END_FINALLY 
 32 LOAD_CONST 0 (None)
 35 RETURN_VALUE 
to
 4 0 LOAD_FAST 0 (l)
 3 DUP_TOP 
 4 LOAD_ATTR 0 (__exit__)
 7 ROT_TWO 
 8 LOAD_ATTR 1 (__enter__)
 11 CALL_FUNCTION 0
 14 POP_TOP 
 15 SETUP_FINALLY 4 (to 22)
 5 18 POP_BLOCK 
 19 LOAD_CONST 0 (None)
 >> 22 WITH_CLEANUP 
 23 END_FINALLY 
 24 LOAD_CONST 0 (None)
 27 RETURN_VALUE 
And speeds it up from:
$ ./python.exe -m timeit -s 'import thread; lock =
thread.allocate_lock()' 'with lock: pass'
1000000 loops, best of 3: 0.832 usec per loop
to:
$ ./python.exe -m timeit -s 'import thread; lock =
thread.allocate_lock()' 'with lock: pass'
1000000 loops, best of 3: 0.762 usec per loop
That's only half of the way to parity with try/finally:
$ ./python.exe -m timeit -s 'import thread; lock =
thread.allocate_lock()' 'lock.acquire()' 'try: pass' 'finally:
lock.release()'
1000000 loops, best of 3: 0.638 usec per loop
What's strange is that calling __enter__ and __exit__ in a try/finally
block brings the speed back to the faster 'with' speed, even though they
call the same C functions:
$ ./python.exe -m timeit -s 'import thread; lock =
thread.allocate_lock()' 'lock.__enter__()' 'try: pass' 'finally:
lock.__exit__()'
1000000 loops, best of 3: 0.754 usec per loop
Any ideas?

Now with documentation, a working test_compile, and one less refleak.

> What's strange is that calling __enter__ and __exit__ in a 
> try/finally block brings the speed back to the faster 'with' speed, 
> even though they call the same C functions
Looking carefully at the code, there are two reasons for this:
- LockType has no methods! try "dir(thread.LockType)". Instead, LockType
defines a tp_getattr which does a *linear* search in a PyMethodDef
array. First items are served faster...
- After converting this to use the usual tp_methods slot, there is still
a performance difference, due to the fact that release() is a
METH_NOARGS method, while __exit__() uses METH_VARARGS.
Phew.

Patch applied cleanly for me and all tests pass.
It also looked good on a visual scan over the diff text.

I went ahead and committed the change to the bytecode generation as r61290.
The deficiencies in the lock implementation should probably be raised as
a separate issue, so I'm closing this one.

Hm, my tests do not see any speedup with this patch.
I used VS2005 on win2K, and VS2008 on winXP.
Timings are very similar before and after this patch.
Maybe the optimization is only useful with gcc?

Thanks Nick and Amaury!
Amaury, what times are you seeing? It could be a just-gcc speedup, but I
wouldn't have thought so since the patch eliminates 2 times around the
eval loop. It could be that the overhead of WITH_CLEANUP is high enough
to drown out those iterations. You had mentioned optimizing the
PyObject_CallFunctionObjArgs() call before. If you're still seeing with
significantly slower than try, that's probably the right place to look.
Here are my current timings. To avoid the lock issues, I wrote
simple_cm.py containing:
class CM(object):
 def __enter__(self):
 pass
 def __exit__(self, *args):
 pass
$ ./python.exe -m timeit -s 'import simple_cm; cm = simple_cm.CM()'
'with cm: pass'
1000000 loops, best of 3: 0.885 usec per loop
$ ./python.exe -m timeit -s 'import simple_cm; cm = simple_cm.CM()'
'cm.__enter__()' 'try: pass' 'finally: cm.__exit__()'
1000000 loops, best of 3: 0.858 usec per loop
If __exit__ doesn't contain *args (making it not a context manager), the
try/finally time goes down to:
$ ./python.exe -m timeit -s 'import simple_cm; cm = simple_cm.CM()'
'cm.__enter__()' 'try: pass' 'finally: cm.__exit__()'
1000000 loops, best of 3: 0.619 usec per loop
I think in theory, with could be slightly faster than finally with the
same argument list, but it's pretty close now.