Re: [Zope] Finding an object in a folder

[Dieter Maurer]

Paul Winkler wrote at 2005-5-27 16:28 -0400:
>On Tue, May 24, 2005 at 08:59:01PM +0200, Dieter Maurer wrote:
>> An incredibly long time ago, I filed a feature request for
>> "hasattr_unacquired" -- together with patch, unit tests and documentation
>> update. 
>
>Do you mean this?
>http://www.zope.org/Collectors/Zope/742

Probably.

>the unit tests and docs are missing.

In what almost surely was the source for the patch,
I see:

--- Products/OFSP/help/dtml-funcs.stx~  2002-01-09 18:44:05.0 +0100
+++ Products/OFSP/help/dtml-funcs.stx   2002-12-21 16:19:49.0 +0100
@@ -55,6 +55,9 @@
 getattr(object, name) and seeing whether it raises an exception or
 not.)
 
+hasattr_unacquired(object, string) -- like "hasattr" but only
+unacquired attributes are taken into account.
+
 hash(object) -- Return the hash value of the object (if it has
 one). Hash values are integers. They are used to quickly compare
 dictionary keys during a dictionary lookup. Numeric values that compare

I.e. the description of "hasattr_unacquired" for Zope's embedded
online help (the only docs Zope contains itself).


Indeed, I did the unit test extension only later (and only to prevent
the test suite to fail -- I trusted my 3 line extension :-).
I added to ".../AccessControl/tests/actual_python.py":

  # After all the above, these wrappers were still untouched:
  # ['DateTime', '_print_', 'reorder', 'same_type', 'test']
+ # DM 2004-07-09: 'hasattr_unacquired' untouched, too
  # So do something to touch them.
  def f9():
+ # DM 2004-07-09: 'hasattr_unacquired'...
+ class C: x=1
+ c = C()
+ assert hasattr_unacquired(c,'x')
+ assert not hasattr_unacquired(c,'xx')
  
  d = DateTime()


A more elaborate test would add:

  from ExtensionClass import Base
  from Acquisition import Implicit

  class B(Base): x=1
  class C(Implicit): y=1
  c = C().__of__(B())
  assert not hasattr_unacquired(c, 'x')
  assert hasattr_unacquired(c, 'y')

-- 
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Re: [Zope] Finding an object in a folder

[Alec Mitchell]

On Saturday 28 May 2005 01:26 am, Dieter Maurer wrote:
> Paul Winkler wrote at 2005-5-27 11:08 -0400:
> >On Fri, May 27, 2005 at 03:26:04PM +0200, Dieter Maurer wrote:
> >> A higher risk comes from the fact that some Python packages may
> >> rely on the broken "hasattr" behaviour. That's the reason why
> >> the problem is not fixed in Python itself.
> >
> >And that's a very good reason not to monkeypatch it.
> >We can't have arbitrary third-party packages breaking when
> >they are used with Zope. Such bugs could be maddeningly difficult
> >to diagnose and find.
> >
> >Anyway, Jim has made a papal edict on zope-dev:
> >We will never monkeypatch hasattr in the zope core.
>
> But hopefully we will get a decent "safe_hasattr" (or similar),
> get it used at least in all Zope core code,
> rather than that each of the 700 occurrencies tries its
> own "hasattr" emulation via "getattr".

Well, Plone at least has one for 2.1 until zope gets one:

from Products.CMFPlone import shasattr

also:

from Products.CMFPlone import safe_callable

shasattr is essentially a copy of the AT one, but is importable into trusted. 
code.

Alec
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Re: [Zope] Finding an object in a folder

[Dieter Maurer]

Paul Winkler wrote at 2005-5-27 11:08 -0400:
>On Fri, May 27, 2005 at 03:26:04PM +0200, Dieter Maurer wrote:
>> A higher risk comes from the fact that some Python packages may
>> rely on the broken "hasattr" behaviour. That's the reason why
>> the problem is not fixed in Python itself.
>
>And that's a very good reason not to monkeypatch it.
>We can't have arbitrary third-party packages breaking when
>they are used with Zope. Such bugs could be maddeningly difficult
>to diagnose and find.
>
>Anyway, Jim has made a papal edict on zope-dev:
>We will never monkeypatch hasattr in the zope core.

But hopefully we will get a decent "safe_hasattr" (or similar),
get it used at least in all Zope core code,
rather than that each of the 700 occurrencies tries its
own "hasattr" emulation via "getattr".

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Re: [Zope] Finding an object in a folder

[Paul Winkler]

On Tue, May 24, 2005 at 08:59:01PM +0200, Dieter Maurer wrote:
> An incredibly long time ago, I filed a feature request for
> "hasattr_unacquired" -- together with patch, unit tests and documentation
> update. 

Do you mean this?
http://www.zope.org/Collectors/Zope/742

the unit tests and docs are missing.
But I like the idea.

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Re: [Zope] Finding an object in a folder

[Paul Winkler]

On Fri, May 27, 2005 at 03:26:04PM +0200, Dieter Maurer wrote:
> A higher risk comes from the fact that some Python packages may
> rely on the broken "hasattr" behaviour. That's the reason why
> the problem is not fixed in Python itself.

And that's a very good reason not to monkeypatch it.
We can't have arbitrary third-party packages breaking when
they are used with Zope. Such bugs could be maddeningly difficult
to diagnose and find.

Anyway, Jim has made a papal edict on zope-dev:
We will never monkeypatch hasattr in the zope core.

-- 

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Re: [Zope] Finding an object in a folder

[Paul Winkler]

On Fri, May 27, 2005 at 01:58:44PM +0200, Florent Guillaume wrote:
> And all empty tuples are equal.

Not just equal, but all empty tuples are *identical*,
so () is () == True. 

I had no idea this was so :-\
 
> In python 2.3, the idiom most commonly found is to use marker = object().

Thanks for that.

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Re: [Zope] Finding an object in a folder

[Dieter Maurer]

Paul Winkler wrote at 2005-5-26 19:57 -0400:
> ...
>> >Make it so! :-)
>> 
>>   _marker = []
>>   def hasattr(obj, attr, marker):

I introduced a bug here (noticed by "Alec Mitchell").

The "marker" argument should have been "_marker=_marker".

> ...
>Are you currently running zope with a patch like this?

No, because I never really met an "hasattr" bug (although
I acknowledge that it may happen).

>Have you noticed any performance difference?
>the zope 2.6 source code contains over 700 calls to hasattr()...

We replace a small C-implemented function with a small Python one.
Although the Python function may be about 10 times slower,
the overall effect is almost surely negligible.

A higher risk comes from the fact that some Python packages may
rely on the broken "hasattr" behaviour. That's the reason why
the problem is not fixed in Python itself.

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Re: [Zope] Finding an object in a folder

[Dieter Maurer]

Paul Winkler wrote at 2005-5-27 05:07 -0400:
> ...
>What's wrong with a tuple? There's no reason to mutate the marker. 

The Python spec allows implementations to identify immutable objects
with the same value. Thus, in principle, two "()" created
at different times can in fact be the same object.
This would violate our use as a marker.

Python specifies that mutable objects created at different times
are garanteed to be different (as long as both exist).
Therefore, you can use them safely as markers.

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Re: [Zope] Finding an object in a folder

[Florent Guillaume]

Paul Winkler  <[EMAIL PROTECTED]> wrote:
> On Fri, May 27, 2005 at 08:51:58AM +0100, Chris Withers wrote:
> > Paul Winkler wrote:
> > 
> > >marker = ()
> > >if getattr(some_object, marker) is not marker:
> > 
> > For good measure, I wonder if that should be a [], no a ()?
> 
> What's wrong with a tuple? There's no reason to mutate the marker. 

The whole point of a marker is that it is unique, so cannot be confused
with a legitimate value of the object examined.

And all empty tuples are equal.

In python 2.3, the idiom most commonly found is to use marker = object().

Florent

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Re: [Zope] Finding an object in a folder

[Paul Winkler]

On Fri, May 27, 2005 at 08:51:58AM +0100, Chris Withers wrote:
> Paul Winkler wrote:
> 
> >marker = ()
> >if getattr(some_object, marker) is not marker:
> 
> For good measure, I wonder if that should be a [], no a ()?

What's wrong with a tuple? There's no reason to mutate the marker. 

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Re: [Zope] Finding an object in a folder

[Chris Withers]

Paul Winkler wrote:


marker = ()
if getattr(some_object, marker) is not marker:


For good measure, I wonder if that should be a [], no a ()?

cheers,

Chris

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Re: [Zope] Finding an object in a folder

[Paul Winkler]

On Thu, May 26, 2005 at 11:52:36PM +0200, Dieter Maurer wrote:
> Chris Withers wrote at 2005-5-25 21:03 +0100:
> > ...
> >> I strongly argue against it. Fix "hasattr" in the Zope context,
> >> instead!
> >
> >Make it so! :-)
> 
>   _marker = []
>   def hasattr(obj, attr, marker):
> a = getattr(obj, attr, _marker)
> return a is not _marker
> 
>   import __builtin__
>   __builtin__.hasattr = hasattr
> 
> Easy enough, isn't it?

Are you currently running zope with a patch like this?
Have you noticed any performance difference?
the zope 2.6 source code contains over 700 calls to hasattr()...

> Now, there need only to be someone who checks it in.

Well, first I think it should be discussed on zope-dev...
unless it already has been? I'll post a message there.
 
-- 

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Re: [Zope] Finding an object in a folder

[Alec Mitchell]

On Thursday 26 May 2005 02:52 pm, Dieter Maurer wrote:
> Chris Withers wrote at 2005-5-25 21:03 +0100:
> > ...
> >
> >> I strongly argue against it. Fix "hasattr" in the Zope context,
> >> instead!
> >
> >Make it so! :-)

I'd add the ability to turn on/off acquisition (off by default, import aq_base 
somewhere above), and get rid of the required marker parameter:

 _marker = []
 def hasattr(obj, attr, acquire=False):
 if not acquire:
 obj = aq_base(obj)
 a = getattr(obj, attr, _marker)
 return a is not _marker

 import __builtin__
 __builtin__.hasattr = hasattr
>
> Easy enough, isn't it?
>
> Way easier than to change "hasattr" anywhere it is now used.
>
>
> Now, there need only to be someone who checks it in.

This would be lovely. :-)

Alec
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Re: [Zope] Finding an object in a folder

[Dieter Maurer]

Chris Withers wrote at 2005-5-25 21:03 +0100:
> ...
>> I strongly argue against it. Fix "hasattr" in the Zope context,
>> instead!
>
>Make it so! :-)

  _marker = []
  def hasattr(obj, attr, marker):
  a = getattr(obj, attr, _marker)
  return a is not _marker

  import __builtin__
  __builtin__.hasattr = hasattr

Easy enough, isn't it?

Way easier than to change "hasattr" anywhere it is now used.


Now, there need only to be someone who checks it in.


>>>if getattr(thefolder.aq_inner.aq_explicit,the_id,None):
>> 
>> You are aware that this is in general *NOT* an emulation
>> of "hasattr".
>
>I didn't say it was ;-)
>
>> It may fail e.g. for properties.
>
>How would it fail for properties?

It fails for everything with a false value (None, 0, '', [], ...).
Properties may have a false value.

>> And it would be so easy to fix "hasattr" ;-)
>
>then why haven't you done it? ;-)

I do not have write access to the Zope repository
(because ZC requires full patent indemnification to grant it
and I am unwilling to promise this).

-- 
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Re: [Zope] Finding an object in a folder

[Phillip Hutchings]

On 27/05/05, Paul Winkler <[EMAIL PROTECTED]> wrote:
> On Fri, May 27, 2005 at 09:20:52AM +1200, Phillip Hutchings wrote:
> > > Well, it doesn't distinguish between an attribute which doesn't exist
> > > and an attribute which has any false value.  This makes it unsuitable as
> > > a hasattr() replacement IMHO.
> >
> > But you're asking if the object posesses an attribute, not any
> > question about the value of the object. I think returning false if the
> > value is false is potentially dangerous - I may want to retrieve a
> > false value.
> 
> "But" what? You just said the same thing that I said :-)

Sorry, shouldn't reply to things at 9am ;)

Anyway, I agree with you, it is broken as the distinction is critical.

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Re: [Zope] Finding an object in a folder

[Paul Winkler]

On Fri, May 27, 2005 at 09:20:52AM +1200, Phillip Hutchings wrote:
> > Well, it doesn't distinguish between an attribute which doesn't exist
> > and an attribute which has any false value.  This makes it unsuitable as
> > a hasattr() replacement IMHO.
> 
> But you're asking if the object posesses an attribute, not any
> question about the value of the object. I think returning false if the
> value is false is potentially dangerous - I may want to retrieve a
> false value.

"But" what? You just said the same thing that I said :-)
 
-- 

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Re: [Zope] Finding an object in a folder

[Phillip Hutchings]

> Well, it doesn't distinguish between an attribute which doesn't exist
> and an attribute which has any false value.  This makes it unsuitable as
> a hasattr() replacement IMHO.

But you're asking if the object posesses an attribute, not any
question about the value of the object. I think returning false if the
value is false is potentially dangerous - I may want to retrieve a
false value.

-- 
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Re: [Zope] Finding an object in a folder

[Paul Winkler]

On Wed, May 25, 2005 at 09:03:13PM +0100, Chris Withers wrote:
> >>if getattr(thefolder.aq_inner.aq_explicit,the_id,None):
> >
> >You are aware that this is in general *NOT* an emulation
> >of "hasattr".
> 
> I didn't say it was ;-)
> 
> >It may fail e.g. for properties.
> 
> How would it fail for properties?

Well, it doesn't distinguish between an attribute which doesn't exist 
and an attribute which has any false value.  This makes it unsuitable as
a hasattr() replacement IMHO.

It feels kind of hacky, but for this kind of thing I've used the "marker" 
pattern that you find throughout the Zope source:

marker = ()
if getattr(some_object, marker) is not marker:
...

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Re: [Zope] Finding an object in a folder

[Chris Withers]

Dieter Maurer wrote:

By the way, it is as nasty in Python as it is in DTML.


I agree with we disagree, I hopethat doesn't cause offence ;-)

Worse still, you use hasattr with ZODB... hastattr swallows all 
exceptions, including ConflictErrors, and especially in this example 
that would be a bad thing...


Again, the correct way to approach this problem is not
to ban "hasattr" from thousands of places
but to monkey patch "__builtin__.hasattr"
such that it behaves in a Zope compatible way.


It doesn't matter what is correct and what isn't correct here, you're 
handing out advice that will break when a person tries it and they least 
expect it to break. That's not a nice thing to do...



I strongly argue against it. Fix "hasattr" in the Zope context,
instead!


Make it so! :-)

if getattr(thefolder.aq_inner.aq_explicit,the_id,None):


You are aware that this is in general *NOT* an emulation
of "hasattr".


I didn't say it was ;-)


It may fail e.g. for properties.


How would it fail for properties?


And it would be so easy to fix "hasattr" ;-)


then why haven't you done it? ;-)

Chris

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Re: [Zope] Finding an object in a folder

[Dieter Maurer]

Chris Withers wrote at 2005-5-23 20:02 +0100:
>Dieter Maurer wrote:
>>>Here is the problem. I want to use a form to upload a file (with a specific 
>>>name) but first I want to check if another file with the same name already 
>>>exist in that folder. How do I use a if or dtml-if to solve this problem??
>> 
>>   
>> "the_id" exists
>>   
>> "the_id" does not exist
>>   
>
>Dieter, I'm shocked ;-)
>
>Why in DTML? Even though Allen mentions DTML, this never belongs in DTML.

We are already familiar with the fact that we often disagree.
This is another case.

   The test whether an object exists is sufficiently trivial
   (and often needed) that it can be in DTML without a
   problem.

An incredibly long time ago, I filed a feature request for
"hasattr_unacquired" -- together with patch, unit tests and documentation
update. I am convinced that such a function in the
DTML namespace (and therefore always available in restricted code)
would be much clearer than the "aq_inner.aq_explicit" dance.

But, unfortunately, the Zope developers decided not to
accept my patch or the "hasattr_unacquired" idea
and instead made "aq_inner" accessible by untrusted code.
A bad decision!
As a consequence, you see the nasty code.

By the way, it is as nasty in Python as it is in DTML.


>Worse still, you use hasattr with ZODB... hastattr swallows all 
>exceptions, including ConflictErrors, and especially in this example 
>that would be a bad thing...

Again, the correct way to approach this problem is not
to ban "hasattr" from thousands of places
but to monkey patch "__builtin__.hasattr"
such that it behaves in a Zope compatible way.

I have seen several severely broken trial to work around
the use of "hasattr" -- by Zope experts not Zope newbies.

I strongly argue against it. Fix "hasattr" in the Zope context,
instead!

>if getattr(thefolder.aq_inner.aq_explicit,the_id,None):

You are aware that this is in general *NOT* an emulation
of "hasattr". It may fail e.g. for properties.

Such half faithful "hasattr" emulations will make
more problems then even the unfixed "hasattr" use.

And it would be so easy to fix "hasattr" ;-)

-- 
Dieter
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Re: [Zope] Finding an object in a folder

[Chris Withers]

Dieter Maurer wrote:

Allen Huang wrote at 2005-5-8 02:28 -0700:


Here is the problem. I want to use a form to upload a file (with a specific 
name) but first I want to check if another file with the same name already 
exist in that folder. How do I use a if or dtml-if to solve this problem??


  
"the_id" exists
  
"the_id" does not exist
  


Dieter, I'm shocked ;-)

Why in DTML? Even though Allen mentions DTML, this never belongs in DTML.

Worse still, you use hasattr with ZODB... hastattr swallows all 
exceptions, including ConflictErrors, and especially in this example 
that would be a bad thing...


if getattr(thefolder.aq_inner.aq_explicit,the_id,None):
   print the_id + ' is in use'
else:
   print the_id + ' is in use'
return printed

cheers,

Chris

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Re: [Zope] Finding an object in a folder

[Dieter Maurer]

Please stay on the list!
Readded...

Allen Huang wrote at 2005-5-8 11:10 -0700:
>Thanks for responding, but what is "aq_inner.aq_explicit"??

To understand "aq_inner" and "aq_explicit", you need
to understand acquisition.

You find something in the "Name lookup" section of

  

and in Jim's acquisition talk (which your probably can
locate via a search for "Jim Fulton acquisition containment before
context").

Once you understood acquisition, you know "aq_self", "aq_parent"
and implicit and explicit acquisition wrappers.
Now, you can understand "aq_inner" and "aq_explicit".

To determine "aq_inner" from an acquisition wrapper,
you follow its "aq_self" recursively, until "aq_self"
is not an acquisition wrapper.

The "aq_explicit" attribute of an acquisition wrapper
returns an explicit acquisition wrapper with the same
"aq_self" and "aq_parent".

-- 
Dieter
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Re: [Zope] Finding an object in a folder

[Dieter Maurer]

Allen Huang wrote at 2005-5-8 02:28 -0700:
>Here is the problem. I want to use a form to upload a file (with a specific 
>name) but first I want to check if another file with the same name already 
>exist in that folder. How do I use a if or dtml-if to solve this problem??

  
"the_id" exists
  
"the_id" does not exist
  


-- 
Dieter
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[Zope] Finding an object in a folder

[Allen Huang]

Here is the problem. I want to use a form to upload a file (with a specific name) but first I want to check if another file with the same name already exist in that folder. How do I use a if or dtml-if to solve this problem??
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