[Tutor] Question on the "object" object

[Daniel Ehrenberg]

Terry Carroll wrote:
> in response to Todd's question in the thread "Do
> tuples replace 
> structures?", I ended up saying:
> 
> > I prefer to use classes, though for just about
> anything where I need a
> > structure:
> >
> > >>> class person:
> > ...   def __init__(self,fname,lname,age,height):
> > ...     self.fname = fname
> > ...     self.lname = lname
> > ...     self.age = age
> > ...     self.height = height
> > ...
> > >>> record = person("Jim", "Smith", 22, 130)
> > >>> record.fname
> > 'Jim'
> 
> This got me wondering; how minimalist can you get in
> a class definition 
> and still be able to access arbitrarily-named
> attributes?  Some toying 
> around to figure that out has gotten me into
> something I don't understand, 
> and would like to.
> 
> Now, none of the following three objects raise an
> Attribute error as a
> result of assigning to foo.bar, a previously
> unreferenced attribute:
> 
> >>> #old-style class
> ... class c1:
> ...     pass
> ...
> >>> o1=c1()
> >>> o1.a="test"
> >>> print "o1", o1.a
> o1 test
> 
> >>> #new-style class
> ... class c2(object):
> ...     pass
> ...
> >>> o2=c2()
> >>> o2.a="test"
> >>> print "o2", o2.a
> o2 test
> 
> >>> #new-style class, calling object's __init__
> ... class c3(object):
> ...     def __init__(self):
> ...         object.__init__(self)
> ...
> >>> o3=c3()
> >>> o3.a="test"
> >>> print "o3", o3.a
> o3 test
> 
> Okay, so if we can do this with an instance of an
> object from a class that 
> inherits from the "object" object, why can't we do
> the same with an 
> instance of the "object" object itself?  (I tried
> several times to phrase 
> that both clearly and accurately, and I'm not sure I
> got there; the 
> example should be clearer.):
> 
> >>> #using object directly, instead of a class
> inherited from it
> ... o4=object()
> >>> o4.a="test"
> Traceback (most recent call last):
>   File "<stdin>", line 1, in ?
> AttributeError: 'object' object has no attribute 'a'
> 
> 
> In sum, why does:
> 
>  x=object()
>  x.a
> 
> raise an AttributeError, when
> 
>  x=c()
>  x.a
> 
> Does note, where c is nothing more than a flat-out,
> no-frills subclass of 
> object?

Object is a type, like int, not a class. Annoyingly,
you can't change anything about these, but they can be
converted into classes quite easily. Here's a small
function and usage examples that can make any builtin
mutable (as long as it's subclassable):
>>> import __builtin__
>>> def mutate(things):
...     for thing in things:
...         class temp(thing): pass
...         setattr(__builtin__, thing.__name__, temp)
...     
>>> mutate([int]) #make ints mutable
>>> x = int(0) #explicitly created
>>> x
0
>>> x.a = 5
>>> x.a
5
>>> y = 0
>>> y.a = 5 #should have been explicitly created int
Traceback (most recent call last):
  File "<input>", line 1, in ?
AttributeError: 'int' object has no attribute 'a'

What you're really trying to do is make this
prototype-oriented. Although it is possible to do that
in Python, it was not really made for that purpose and
so it doesn't work well. Python doesn't have good
anonymous functions (lambdas only evaluate something,
they can't have statements or flow control), so if you
want to use prototypes more, you might want to try a
language with perl-style object orientation (such as
Lua, Javascript, Nasal, etc), one that's completely
prototype-oriented (such as Io or Newtonscript), or a
overly-pure object oriented language (such as Ruby or
Smalltalk), which seem to inadvertantly support
prototype-orientation. I've looked at these, since
prototype-orientation seems more flexible, they turn
out to be less either flexible for everything else or
too minimalistic.

In my opinion, the best solution to the original
problem of datastructures would be best solved using
dictionaries, because that's what they were intended
for and they work well for that.

Daniel Ehrenberg

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