Re: [Zope-dev] Re: hasattr implementation for Zope?

2005-05-28 Thread Dieter Maurer
Tres Seaver wrote at 2005-5-27 08:22 -0400:
 ...
As a local patch, this isn't too bad (one could even package it as a
do-nothing-after-initialization product).  However, no redistributed
product code should rely on the presence of a patched 'hasattr', but
should use the 3 argument getattr instead.

I think this is *very* bad!

  It is quite difficult to get the hasattr emulation via
  getattr correct.

If you do not like a modified __builtin__.hasattr (which I can understand),
then provide at least a __builtin__.zhasattr (or similarly named) available
in restricted code and strongly recommend in the documentation to use this
variant of hasattr for any Zope/ZODB object.

-- 
Dieter
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Re: [Zope-dev] Re: hasattr implementation for Zope?

2005-05-28 Thread Dieter Maurer
Jim Fulton wrote at 2005-5-27 11:49 -0400:
 ...
I'm sure this was an unintentional non-acceptance.  It would be
a lot easier if Dieter became a contributor and checked this in
himself.

You know the unqualified indemnification clause concerning
patents it preventing me...

-- 
Dieter
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Re: [Zope-dev] Re: hasattr implementation for Zope?

2005-05-28 Thread Dieter Maurer
Paul Winkler wrote at 2005-5-27 11:02 -0400:
 ...
def safe_hasattr(obj, attr, acquired=True, _marker=[]):
if not acquired:
obj = aq_inner(aq_explicit(obj))

This should be obj = aq_base(obj).

The aq_explicit(aq_inner(...)) dance is only necessary in
untrusted code as aq_base is not available there.

-- 
Dieter
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Re: [Zope-dev] RAMcache and container vs. context

2005-05-28 Thread Dieter Maurer
Stefan H. Holek wrote at 2005-5-27 10:59 +0100:
A TALES expression may be prohibitively expensive in any case, no  
matter how simple it is kept. Please make sure to do some comparative  
profiling. Cache keys are recomputed on every call of the script,  
AFAICS. The thought of doing this in restricted Python makes my skin  
crawl.

But TALES expressions need not necessarily be restricted (see my
TrustedExecutables).

Though, I did not think about the security inplications
when we would use unrestricted TALES expressions for a cache manager.
It would probably be a bad idea.

-- 
Dieter
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Re: [Zope-dev] Re: hasattr implementation for Zope?

2005-05-28 Thread Dieter Maurer
Tim Peters wrote at 2005-5-27 13:49 -0400:
 ...
def lookup1(arg, _marker=object()):
return _marker
 ...
_marker = object()
def lookup3(arg):
return _marker
 ...
lookup1  0.427597
lookup3  0.404399

Do you understand why lookup3 is faster than lookup1?

I had the impression that access to the function's local namespace
should be faster than any other variable access.

-- 
Dieter
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Re: [Zope-dev] Re: hasattr implementation for Zope?

2005-05-28 Thread Tim Peters
[Tim Peters]
 def lookup1(arg, _marker=object()):
 return _marker
 ...
 _marker = object()
 def lookup3(arg):
 return _marker
 ...
lookup1  0.427597
lookup3  0.404399

[Dieter Maurer] 
 Do you understand why lookup3 is faster than lookup1?

 I had the impression that access to the function's local namespace
 should be faster than any other variable access.

It is, although module-global lookup is often much faster now than
Python old-timers know it is wink:  the critical successful path
thru the dict lookup code for a dict keyed by interned strings is, in
the absence of collisions, _almost_ as lean as an indexed array access
now.  The primary difference in speed remaining is that the dict
lookup endures an extra C-level function call.

I explained a relevant difference last time:  lookup1 has the
additional expense of initializing an additional local variable. 
That's what a default argument becomes, and nothing happens for free. 
Let's time this effect in isolation:


from itertools import repeat
from time import clock as now

def with(default=5):
pass

def without():
pass

for dummy in range(3):
for f in with, without:
start = now()
for dummy in repeat(None, 100):
   f()
finish = now()
print %-8s %.6g % (f.__name__, finish - start)


and typical output:

with 0.374435
without  0.338555
with 0.370469
without  0.339791
with 0.372165
without  0.337355

So adding a default argument indeed has a measurable cost (although
~0.03 seconds across a million calls is nothing to fret about!).  Note
that the difference between lookup1 and lookup3 was less than that on
this box, reflecting that the lookup _inside_ lookup1 goes faster than
the lookup inside lookup3, partly cancelling lookup1's higher call
overhead; the difference in call overhead is greater than the
difference in lookup speed, so lookup1 ends up a net loser.
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Re: [Zope-dev] Re: hasattr implementation for Zope?

2005-05-28 Thread Tres Seaver
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Hash: SHA1

Dieter Maurer wrote:
 Tres Seaver wrote at 2005-5-27 08:22 -0400:
 
...
As a local patch, this isn't too bad (one could even package it as a
do-nothing-after-initialization product).  However, no redistributed
product code should rely on the presence of a patched 'hasattr', but
should use the 3 argument getattr instead.
 
 
 I think this is *very* bad!
 
   It is quite difficult to get the hasattr emulation via
   getattr correct.
 
 If you do not like a modified __builtin__.hasattr (which I can understand),
 then provide at least a __builtin__.zhasattr (or similarly named) available
 in restricted code and strongly recommend in the documentation to use this
 variant of hasattr for any Zope/ZODB object.

As Tim notes, if we aren't replacing the builtin, then we could equally
well recommend the following pattern:

  # in ZODB.utils

  def safe_hasattr(obj, name, _marker=object()):
  Make sure we don't mask exceptions like hasattr().

  We don't want exceptions other than AttributeError to be masked,
  since that too often masks other programming errors.
  Three-argument getattr() doesn't mask those, so we use that to
  implement our own hasattr() replacement.
  
  return getattr(obj, name, _marker) is not _marker

   in any module which uses hasattr
  from ZODB.utils import safe_hasattr
  
  # was:  if hasattr(object, 'attribute'):
  if safe_hasattr(object, 'attribute'):

The spelling, of course, would be up for endless argument. ;)  Perhaps
we could call it 'persistent_hasattr', and then let folks alias it at
import time as they wish (even to 'hasattr', but not in the core!).

We would then (as you note) need to ensure that we made it available to
restricted code.  For that casee, we might need a variant of it which
used 'guarded_getattr' (but I'm not sure about that).



Tres.
- --
===
Tres Seaver   [EMAIL PROTECTED]
Palladion Software   Excellence by Designhttp://palladion.com
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