Need a better understanding on how MRO works?

[Steven W. Orr]

On Saturday, Aug 25th 2007 at 17:19 -0700, quoth Alex Martelli:

=>Steven W. Orr <steveo at syslang.net> wrote:
=>   ...
=>>      name = 'C1'
=>>      nclass = new.classobj(name,(D1,),globals())
=>>      globals()[name] = nclass
=>
=>Here, you're creating a VERY anomalous class C1 whose __dict__ is
=>globals(), i.e. the dict of this module object;
=>
=>>      name = 'C2'
=>>      nclass = new.classobj(name,(D1,),globals())
=>>      globals()[name] = nclass
=>
=>and here you're creating another class with the SAME __dict__;
=>
=>>      globals()[name].m1 = m1replace
=>
=>So of course this assignment affects the 'm1' entries in the dict of
=>both classes, since they have the SAME dict object (a la Borg) -- that
=>is, IF they're old-style classes (i.e. if D1 is old-style), since in
=>that case a class's __dict__ is in fact a dict object, plain and simple.
=>
=>However, if D1 is new-style, then C1.__dict__ and C2.__dict__ are in
=>fact instances of <dictproxy> -- each with a copy of the entries that
=>were in globals() when you called new.classobj, but DISTINCT from each
=>other and from globals(), so that further changes in one (or globals)
=>don't affect globals (nor the other).
=>
=>I guess this might be a decent interview question if somebody claims to
=>be a "Python guru": if they can make head or tails out of this mess, boy
=>the *ARE* a Python guru indeed (in fact I'd accord minor guruhood even
=>to somebody who can get a glimmer of understanding of this with ten
=>minutes at a Python interactive prompt or the like, as opposed to
=>needing to understand it "on paper" without the ability to explore:-).
=>
=>Among the several "don't"s to learn from this: don't use old-style
=>classes, don't try to make two classes share the same dictionary, and
=>don't ask about MRO in a question that has nothing to do with MRO
=>(though I admit that was a decent attempt at misdirection, it wouldn't
=>slow down even the minor-guru in any appreciable way:-).
=>
=>
=>Alex
=>-- 
=>http://mail.python.org/mailman/listinfo/python-list
=>

Thanks Alex. I am humbled, though I was before I started.
I really don't have a lot of understanding of what you're saying so I'll 
probably have to study this for about a year or so.

* (I need to look up what dictproxy is.) I don't have any idea what the 
ramifications are of your use of the word DISTINCT. Are you somehow 
suggesting that new.classobj does a deep copy of the globals copy that's 
passed to it?

* Also, I'd like to understand what the difference is between 
    nclass = new.classobj(name,(D1,),globals())
vs. 
    def classfactory():
	class somename(object):
	    def somestuff():
		pass
        return somename
    G1 = classfactory()
    globals()[name] = G1

Does new.classobj do anything special?

I appreciate your time. 

-- 
Time flies like the wind. Fruit flies like a banana. Stranger things have  .0.
happened but none stranger than this. Does your driver's license say Organ ..0
Donor?Black holes are where God divided by zero. Listen to me! We are all- 000
individuals! What if this weren't a hypothetical question?
steveo at syslang.net