[Python-bugs-list] [ python-Bugs-672115 ] Assignment to __bases__ of direct object subclasses

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Bugs item #672115, was opened at 2003-01-21 22:45
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Category: Python Interpreter Core
Group: Python 2.3
Status: Open
Resolution: None
Priority: 5
Submitted By: Greg Chapman (glchapman)
Assigned to: Michael Hudson (mwh)
Summary: Assignment to __bases__ of direct object subclasses

Initial Comment:
I'm not entirely sure this is a bug, but I think it is 
surprising:

Python 2.3a1 (#38, Dec 31 2002, 17:53:59) [MSC 
v.1200 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more 
information.
>>> class A(object):
...     pass
...
>>> class B(object):
...     pass
...
>>> B.__bases__ = (A,)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: __bases__ assignment: 'A' deallocator differs 
from 'object'

It seems like you should be able to change the 
__bases__ of a new-style class (implemented in 
Python) which inherits directly from object to another 
new-style class.  (Will the deallocator issue ever come 
into play if the only types involved are HEAPTYPES and 
object as the ultimate base?)


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>Comment By: Michael Hudson (mwh)
Date: 2003-01-31 18:09

Message:
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Are you interested in working up a patch for this?  Hacking
this kind of stuff requires motivation I'm not sure I can
drum up in time for 2.3...

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Comment By: Greg Chapman (glchapman)
Date: 2003-01-23 17:03

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Sorry about the parenthetical comment; I think what I was trying 
to say is basically what you have in your last paragraph.

As for use cases, I don't have any myself (I ran into this with 
some test code for a metaclass which "overrides" __bases__).  
However, grepping through the standard library, I note that one 
place where assignment to __bases__ is used is in 
xmlrpclib.SlowParser.  It appears to me that if SlowParser and 
xmllib.XMLParser (neither of which has a base class) were 
converted to new-style classes, the assignment to __bases__ 
would generate this exception.  Of course, that shouldn't be too 
hard to work around if that turns out to be necessary.



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Comment By: Michael Hudson (mwh)
Date: 2003-01-22 11:50

Message:
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I agree this is a bit surprising.  When I was writing this
code I went for the conservative-as-possible approach as I
didn't want to introduce instabilities to Python.

It certainly may be that I've overdone it.  In this case I
probably have; if the tp_dealloc of the class being adjusted
is subtype_dealloc and the tp_dealloc that ultimately gets
invoked is the same we're probably OK.  But I'm not sure and
it's been a while since I thought about this.

It also happens that my motivating use-case for this isn't
troubled by this restriction.

I don't understand your last, parenthetical, comment. 
HEAPTYPES as such doesn't come into it, does it?  

You might be right that we don't need to worry about
tp_dealloc if the ultimate solid_base doesn't change and all
the tp_deallocs on the way there are subtype_dealloc...

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https://sourceforge.net/tracker/?func=detail&atid=105470&aid=672115&group_id=5470