[Python-bugs-list] [ python-Bugs-643841 ] New class special method lookup change

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Bugs item #643841, was opened at 2002-11-25 18:37
You can respond by visiting: 
https://sourceforge.net/tracker/?func=detail&atid=105470&aid=643841&group_id=5470

Category: None
Group: None
Status: Open
Resolution: None
Priority: 5
Submitted By: Terry J. Reedy (tjreedy)
Assigned to: Nobody/Anonymous (nobody)
Summary: New class special method lookup change

Initial Comment:

The lookup of special methods invoked implicitly by 
syntax other than explicit instance.attribute changed 
to *not* use __gettattr__ when normal lookup failed.  
This is contrary to docs, which consistently say 
__getattr__ is unchanged.  New special method 
__getattribute__ is also bypassed, contrary to 
implication of doc-essay.

This was reported on c.l.p by Jan Decaluwe using 
2.2.2 on Red Hat Linux.  On Win98, 2.2.1, I get same 
behavior (I added test of __getattribute__):

class Iold:
  def __init__(self,ob):
    self.ob = ob
  def __getattr__(self,name):
    return getattr(self.ob,name)

class Inew(object):
  def __init__(self,ob):
    self.ob = ob
  def __getattr__(self,name):
    return getattr(self.ob,name)

a = Iold(1) #2
b = Inew(1) #2
a.__add__(1) #2
b.__add__(1) #2
a+1 #2
b+1 #error
#TypeError: unsupported operand types for +: 'Inew' 
and 'int'
Inew.__getattribute__ = Inew.__getattr__
b+1 #same error, no loop
#TypeError: unsupported operand types for +: 'Inew' 
and 'int'

b.__add__(1) # WARNING starts 'infinite' loop

def _(self,other): print 'hello'

Inew.__add__ = _
b+1 #prints 'hello', __getattribute__ bypassed.

http://python.org/2.2/descrintro.html says:
"Note that while in general operator overloading 
works just as for classic classes, there are some 
differences. (The biggest one is the lack of support for 
__coerce__; new-style classes should always use the 
new-style numeric API, which passes the other 
operand uncoerced to the __add__ and __radd__ 
methods, etc.) "

Was lookup change meant to be one of differences?

"There's a new way of overriding attribute access. The 
__getattr__ hook, if defined, works the same way as it 
does for classic classes: it is only called if the regular 
way of searching for the attribute doesn't find it."

But it is different.

 "But you can now also override __getattribute__, a 
new operation that is called for all attribute 
references." 

Except for implicit special methods.

I did not classify discrepancy because I don't know 
whether interpreter or docs are off.






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