obj.__dict__ expected behavior or bug?

[Erik Max Francis]

Ed Young wrote:

> What happened to c.a, c.b, and c.c when iterating thru
> c.__dict__ ?
> 
> It appears that __dict__ members are not instantiated
> until they are changed.
> 
> This precludes using __dict__ as the dictionary in
> a formatted print statement.  e.g.
> 
> print "c.a=%(a)s, c.b=%(b)s, c.c=%(c)s" % c.__dict__
> 
> Is this a bug or expected behavior?

Expected behavior.  What you're missing is the general way that Python
does attribute lookup.  When c is an instance and you say c.x, Python
looks in c's __dict__ for an 'x' entry, then it looks in c's class's
__dict__ for an 'x' entry, then it looks (in a well-defined way) through
c's class's base classes, if any, for an 'x' entry in their __dict__
members.

When you defined

	class C:
	    a = ...
	    b = ...

and so on, these are all _class_ attributes.  When you instantiate a C
and then wrote self.a = ... in its methods, you instantiated _instance_
attributes on that instance.  Class attributes are analogous to static
members/fields in other languages:

>>> class C: # class with two class attributes
...  a = 1
...  b = 2
... 
>>> c = C()
>>> d = C()
>>> c.a
1
>>> d.a
1
>>> c.a = 10 # change an instance attribute
>>> c.a
10
>>> d.a
1
>>> C.b = 20 # change a class attribute
>>> c.b
20
>>> d.b
20
>>> C.__dict__
{'a': 1, '__module__': '__main__', 'b': 20, '__doc__': None}
>>> c.__dict__
{'a': 10}
>>> d.__dict__
{}

-- 
   Erik Max Francis && max at alcyone.com && http://www.alcyone.com/max/
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