[Zope-dev] Re: "hasattr" geddon

[Lennart Regebro]

Shane Hathaway wrote:
> On Friday 09 July 2004 11:50 am, Casey Duncan wrote:
> 
>>On Fri, 9 Jul 2004 16:22:17 +0200
>>Dieter Maurer <dieter at handshake.de> wrote:
>>[..]
>>
>>
>>>The "hasattr" replacement in Python's "__builtin__" could look like:
>>>
>>>    _marker = []
>>>    def hasattr(obj, attr):
>>>	return getattr(obj, attr, _marker) is not _marker
> 
> 
> Hmm.  I just heard about this "hasattr geddon".  hasattr is *good*.  Why are 
> we fixing hasattr and bare excepts when the real problem is ZODB?  ZODB 
> should *not* be sensitive to the way the application handles ConflictErrors.  
> When a ConflictError (or ReadConflictError) occurs, the transaction should 
> fall into an "uncommitable" state.  From this state, you can only abort the 
> transaction; any attempts to write an object or commit cause another 
> ConflictError.  Then, only code that can guarantee that the attempted 
> transaction is complete should actually abort the transaction, and 
> fortunately ZPublisher fits that role.  Today, the abort is implicit, and 
> that's the mistake that has caused us to litter the code with knowledge of 
> ConflictErrors.  With the "uncommitable" state, it would not matter if the 
> application swallows ConflictErrors.
> 
> That said, I was surprised to discover that Python 2.3 implements hasattr this 
> way (from bltinmodule.c):
> 
>         v = PyObject_GetAttr(v, name);
>         if (v == NULL) {
>                 PyErr_Clear();
>                 Py_INCREF(Py_False);
>                 return Py_False;
>         }
> 	Py_DECREF(v);
> 	Py_INCREF(Py_True);
> 	return Py_True;
> 
> It should not swallow all errors, especially now that descriptors make 
> computed attributes quite common.  getattr() only recently started catching 
> only AttributeErrors, but apparently hasattr is lagging behind.  I suggest 
> the consistency between getattr and hasattr should be fixed in Python, not 
> Zope.

Guido rejected that idea 18 months ago. I think he is wrong, and after 
som discussion on #python-dev today (which practically enough was a 
python bug day) there seemed to be some support for this there too. It 
might be worth raising this on the python-dev mailing list. The main 
reason for not doing this change seems to be that it would break too 
much code. Here we have a case where lot's of code IS broken and will be 
fixed. ;)

If this doesn't happen in Python 2.4, I think Zope patching this is the 
best solution. It is natural to use hasattr, and it will be very hard to 
get the whole zope-community to stop using it.

https://sourceforge.net/tracker/index.php?func=detail&aid=504714&group_id=5470&atid=305470