[Jython-bugs] [ jython-Bugs-1676293 ] metaclass __init__ dct reference

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Bugs item #1676293, was opened at 2007-03-08 08:09
Message generated for change (Comment added) made by h_eriksson
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Category: Core
Group: None
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: Henrik Eriksson (h_eriksson)
Assigned to: Nobody/Anonymous (nobody)
Summary: metaclass __init__ dct reference

Initial Comment:
The dct object that gets passed to a metaclass' __init__ method seems to point to the actual __dict__ of the class beeing created. I guess it should be a copy?

The following code breaks with a KeyError since delattr removes the attribute from dct.

def __init__(cls, name, bases, dct):
  # ...snip
  dctnames = dct.keys()
  if name.endswith("__doc"):
    if hasattr(cls, name):
      delattr(cls, name)
                
  val = dct[name]
  # ...snip

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>Comment By: Henrik Eriksson (h_eriksson)
Date: 2007-03-08 11:59

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As you (Samule) say this seems like an obscure implementation detail and
I'm not sure it should be fixed... I could try reasoning with the CP folks
and see if they can fix it in their code instead.

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Comment By: Henrik Eriksson (h_eriksson)
Date: 2007-03-08 11:46

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Sorry for the confusing description. I should have referred to the
mail-thread about "metaclasses in jython" on jython-dev (Charlie asked me
to post the bug). 

Hehe... the code is from CherryPy 3.

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Comment By: Samuele Pedroni (pedronis)
Date: 2007-03-08 11:21

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now I see, there is indeed a copy involved in CPython, but is what is
stored on the type which is a copy.

This is honestly an obscure implementation detail, and not copying is
faster. Easy enough to do the CPython thing tough.

Still whoever wrote that sort of code that depends on this should feel
dirty :) .

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Comment By: Samuele Pedroni (pedronis)
Date: 2007-03-08 10:47

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why guessing when you can check?  I still don't understand what's the
problem, too little information

Python 2.4.3 (#1, Apr  7 2006, 10:54:33) 
[GCC 4.0.1 (Apple Computer, Inc. build 5250)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> class T(type): 
...    def __init__(self, name, bases, dct):
...        global d 
...        d = dct
... 
>>> class X(object):
...    __metaclass__ = T
...    global d1
...    d1 = locals()
... 
>>> d1 is d
True


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